WASHINGTON — President Donald Trump ordered a military attack against Syrian President Bashar Assad on Friday, joining allies Britain and France in launching missile strikes in retaliation for what Western nations said was the deliberate gassing of Syrian civilians.

The coordinated strike marked the second time in a year that Trump has used force against Assad, who U.S. officials believe has continued to test the West’s willingness to accept gruesome chemical attacks.

Trump, speaking from the White House late Friday, said the attack last weekend was “a significant escalation” of Assad’s use of chemical weapons and warranted a stepped-up international response.

The alleged chemical weapons use was not the work of “a man,” Trump said. They were “the crimes of a monster instead.”

Trump said the mandate for an allied attack was open-ended, but Pentagon chiefs later said the strikes Friday would be repeated only if Assad took further action that warranted a response.

Gen. Joseph Dunford Jr., chairman of the Joint Chiefs of Staff, said the U.S. military, in conjunction with British and French forces, struck three sites – a scientific research center near Damascus, a chemical weapons storage facility near Homs and a storage facility and command post also near Homs.

Dunford said that unlike the unilateral U.S. strike in Syria last year, in which only one site was attacked, the United States worked with two allies and hit the three sites in an operation that he said would result in the long-term degradation of Syria’s ability to research, develop and deploy chemical weapons.

Defense Secretary Jim Mattis declined to say whether he thought the attack would prevent Assad from using chemical weapons again.

“Nothing is certain in these kinds of matters. However, we used a little over double the number of weapons this year than we used last year,” he said. “It was done on targets that we believed were selected to hurt the chemical weapons program. We confined it to the chemical weapons-type targets.”

Mattis said that to his knowledge there were no U.S. or allied losses from the strikes Friday.

Dunford said that the only communications that took place between the United States and Russia before the operation were “the normal deconfliction of the airspace, the procedures that are in place for all of our operations in Syria.”

It was not immediately clear how the Syrian military responded to the attack. Dunford said that Syrian forces fired surface-to-air missiles but that he did not have a full picture of the response. He said the Pentagon would provide more details Saturday.

British Prime Minister Theresa May issued a statement saying the attacks were a response to “circumstances of pure horror.”

In a statement, French President Emmanuel Macron said, “Our response has been limited to the Syrian regimes facilities enabling the production and deployment of chemical weapons.”

Vice President Mike Pence left the opening ceremony of the Summit of the Americas in Lima, Peru, to place calls to all four congressional leaders in advance of the airstrikes, speaking directly to Senate Majority Leader Mitch McConnell, R-Ky., House Speaker Paul Ryan, R-Wis., and House Minority Leader Nancy Pelosi, D-Calif.; a spokesman for Senate Minority Leader Charles Schumer, D-N.Y., said that he was also notified just prior to Trump’s statement.

The assault followed repeated threats of military action from Trump, who has been moved by civilian suffering to set aside his concerns about foreign military conflicts, since the reported chemical attack that killed civilians in a rebel-held town outside Damascus last weekend.

The operation capped nearly a week of debate in which Pentagon leaders voiced concerns that an attack could pull the United States into Syria’s civil war and trigger a dangerous conflict with Assad ally Russia – without necessarily halting chemical attacks.

Both Syria and Russia have denied involvement in the attack, which Russian Foreign Minister Sergei Lavrov alleged had been staged.

The episode is the latest illustration of the hazards arising from a conflict that has killed an estimated half-million people and drawn in world powers since it began as peaceful protests in 2011.

The attack raised the possibility of retaliation by Russia or Iran, which also provides military support to Assad, threatening in particular to increase the risks facing a force of 2,000 Americans in Syria as part of the battle against the Islamic State. While the United States has not been at war with the Syrian government, U.S. troops often operate in proximity to Iranian- or Russian-backed groups.

In the wake of last weekend’s gruesome attack, some U.S. officials advocated a larger, and therefore riskier, strike than the limited action Trump ordered in April 2017, also in response to suspected chemical weapons use.

That attack involved 59 Tomahawk missiles fired from two U.S. warships in the Mediterranean Sea. It fulfilled Trump’s vow that chemical weapons are a “red line” that he, unlike his predecessor Barack Obama, would not allow Assad to cross. But the airfield targeted by the Pentagon resumed operations shortly after the attack and, according to Western intelligence assessments, chemical attacks resumed.

Assad’s defiance has presented Trump with a choice of whether to make a larger statement and incur a larger risk this time. Planning for these strikes focused on ways to curb Assad’s ability to use such weapons again.

Risks of a wider attack include the possibility of a dangerous escalation with Russia, whose decision to send its military to Syria in 2015 reversed the course of the war in Assad’s favor. Since then, Russia has used Syria as a testing ground for some of its most sophisticated weaponry.

“Get ready Russia, because they will be coming, nice and new and ‘smart!’ ” Trump tweeted Wednesday, referring to U.S. missiles.

That took military officials by surprise. But on Thursday, Trump said he did not mean to suggest missile strikes were imminent.

“Never said when an attack on Syria would take place,” he tweeted. “Could be very soon or not so soon at all!”

Since last year’s strike, multiple chemical attacks have been reported in opposition areas, most of them involving chlorine rather than the nerve agent sarin, as was used in 2017, suggesting the government may have adjusted its tactics.

Earlier Friday, Nikki Haley, the U.S. ambassador to the United Nations, accused Russia of lying and covering up for the Assad government. Assad’s government had used chemical weapons at least 50 times in the past seven years of warfare, Haley claimed.

“Russia can complain all it wants about fake news, but no one is buying its lies and its coverups,” she said. “Russia was supposed to guarantee Assad would not use chemical weapons, and Russia did the opposite.”

Russia had called for the emergency U.N. Security Council meeting on Syria as military action seemed likely.

Russia’s U.N. ambassador, Vassily Nebenzia, had accused the United States, France and Britain of saber-rattling.

“Why are you seeking to plunge the Middle East into such difficulties, provoking one conflict after another, pitting one state against another?” he said, claiming that anti-government militias had received “instructions” to begin an offensive as soon as an act of force begins. “Is the latest wave of chaos being unleashed only for the sake of that?”

Russia is Assad’s most powerful ally and has thousands of troops and military advisers, as well as air defense systems, deployed in Syria.

Russia’s military has threatened to shoot down any U.S. missiles that put Russian lives at risk. Russia could also fire at the launch platforms used – potentially U.S. planes or ships. Russian officials have said U.S. and Russian military staffs remain in contact regarding Syria, even as Russian media have carried stories in recent days about the potential outbreak of “World War III” as a consequence of a U.S. airstrike against Assad.

Russian President Vladimir Putin warned Macron in a phone call Friday the situation remained tense, the Kremlin said in a statement.

“Most important, it is imperative to avoid badly planned and dangerous actions that would be crude violations of the U.N. Charter and would have unpredictable consequences,” the Kremlin said. “Both leaders directed the ministers of defense and foreign affairs to maintain close contact with the goal of de-escalating the situation.”

U.N. Secretary General António Guterres told the Security Council that he feared events could escalate rapidly into a regional and even global conflict, and urged all states “to act responsibly in these dangerous circumstances.”

France’s U.N. ambassador, Francois Delattre, said the Syrian government’s decision to use chemical weapons meant that it had “reached a point of no return,” necessitating a “robust, united and steadfast response.”

“France will shoulder its responsibility to end an intolerable threat to our collective security,” Delattre told the Security Council.

British U.N. Ambassador Karen Pierce noted that May’s Cabinet “has agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime.”

Announcement of that approval Thursday did not specify that the response should be military, although that was the expectation.

“We will continue to work with our friends and allies to coordinate an international response to that end,” Pierce said Friday.

Opposition lawmakers urged May to first seek Parliament’s consent before committing to any military action. Nothing requires that May do so, but the convention is for British lawmakers to be given the chance to vote. Parliament is in recess but could be recalled for an emergency session.

Meanwhile, a team of investigators from the Organization for the Prohibition of Chemical Weapons arrived in Syria to look for evidence.

The Washington Post’s Carol Morello and Paul Sonne contributed to this report.