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 announced the strikes in an address to the nation Friday evening.

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 the 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.

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 a peaceful uprising 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 those from Iranian- or Russian-backed groups.

Trump said Friday night, “To Iran and to Russia, I ask: What kind of a nation wants to be associated with the mass murder of innocent men, women and children?”

He said the two countries those “most responsible for supporting, equipping and financing the criminal Assad regime.”

“The nations of the world can be judged by the friends they keep,” he said.

He added ominously, “Hopefully someday we’ll get along with Russia, and maybe even Iran, but maybe not.”

(Hassan Ammar | The Associated Press) Smoke rises at sunrise after airstrikes targeting different parts of the Syrian capital Damascus, early Saturday, April 14, 2018. Syria's capital has been rocked by loud explosions that lit up the sky with heavy smoke as U.S. President Donald Trump announced airstrikes in retaliation for the country's alleged use of chemical weapons.

Syria’s capital was rocked by loud explosions that lit up the sky with heavy smoke shortly after the televised announcement.

Associated Press reporters saw smoke rising from eastern Damascus early Saturday morning local time. Syrian state TV said the attack had begun, though it wasn’t immediately clear what was targeted.

Republican Rep. Chris Stewart and other Utahns in Congress supported the coalition’s airstrike. The former Air Force pilot said the attack was “the right thing” to do.

Sen. Orrin Hatch, R-Utah, said on Twitter that Trump’s decision reflects “his seriousness in addressing the scale and depravity of Assad’s actions.”

Hatch’s Utah colleague in the Senate, Mike Lee, questioned whether the U.S. attack was the start of a long-term campaign.

Rep. John Curtis, Utah’s newest congressional delegate also responded to Trump’s announcement.

Rep. Mia Love called Assad’s actions “atrocious human rights violations.”

“We stand together as a nation,” she said in an email. “God bless our brave servicemen and women who are in harm’s way and serving our country.”

Defense Secretary James Mattis said in his own news conference Friday night that no additional strikes on Syria were planned: “Right now, this is a one-time shot.”

Map locates the capital of Syria, where U.S. and ally strikes were reported in retaliation of Syria’s use of chemical weapons.

The strikes in Syria are “directed at the Syrian regime,” he said, and the military has “gone to great lengths to avoid civilians and foreign casualties.”

The U.S. had no reports of injuries or losses in the attack, Mattis said. The Pentagon, he said, would provide more information about the strike on Saturday.

In France, President Emmanuel Macron said Friday night that a “red line has been crossed,” referencing the chemical attack.

There is “no doubt,” he said, that the Syrian government is responsible.

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 had 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.

The British defense ministry says “initial indications” show that the airstrikes against Syria produced a “successful attack” on a Syrian military facility.

(UK Government via AP Video) In this image made from video provided by UK Government via AP Video, British Prime Minister Theresa May speaks in London, Saturday, April 14, 2018. In a message Prime Minister May announced early on Saturday morning that she has authorised British armed forces to "conduct coordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability and deter their use."

British Prime Minister Theresa May described the attack as neither “about intervening in a civil war” nor “about regime change,” but a limited and targeted strike that “does not further escalate tensions in the region” and does everything possible to prevent civilian casualties.

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!”

A larger strike, possibly including stealth aircraft and strikes on multiple sites, could inflict lasting damage to military facilities and economic infrastructure that have been vital to Assad’s ability to gain the upper hand in a seven-year civil war.

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.

Among the chief factors military planners must consider are air defenses in Syria, which were bolstered by Russia’s decision to enter the war in 2015 and could pose a threat should the Pentagon employ manned aircraft in the attack. Their reach was demonstrated in February when an Israeli F-16 fighter jet crashed amid Syrian antiaircraft fire.

The United States has flown an array of aircraft over Syria since it began strikes against the Islamic State in 2014, but those operations have mostly steered clear of government and Russian activities. The Assad regime has not authorized the U.S. operations, but it also has not tried to shoot down American aircraft.

Trump said in his address Friday that he has asked U.S. partners asked U.S. partners “to take greater responsibility for securing their home region, including contributing large amounts of money for the resources, equipment and all of the anti-ISIS effort.”

He said increased engagement from countries including Saudi Arabia, the United Arab Emirates, Qatar and Egypt can ensure that Iran does not profit from the defeat of the Islamic State group.

He added: “America does not seek an indefinite presence in Syria — under no circumstances” and says that, “As other nations step up our contributions, we look forward to the day when we can bring our warriors home.”

— The Associated Press contributed to this report.