BEIRUT — Four Americans were killed in a suicide attack in Syria on Wednesday, the largest loss of life in the Pentagon’s war against Islamic State militants there and a sign of the potent threat that remains as the Trump administration begins to withdraw troops.
Officials said a bomber detonated an explosive vest as a group of Americans, including two service members, a Pentagon civilian, and a US contractor slain in the attack, met with local military officials at a restaurant in the northern city of Manbij.
Three additional US service members were wounded, US Central Command said in a statement.
The incident occurs as the Pentagon begins its drawdown from Syria in keeping with President Trump’s surprise announcement last month that the Islamic State had been defeated and troops would be coming home.
That shift upended plans, backed by military leaders and Trump’s top national security advisers, for an ongoing mission in Syria and drew widespread criticism, including from Republican allies who warned a premature departure could allow militants to return. Nearly a month after Trump’s initial pronouncement, conflicting statements from senior officials, including the president himself, have fueled ongoing confusion about what precisely the administration’s plan entails.
The Islamic State, in a message posted by its unofficial news agency, Amaq, asserted responsibility for the Manbij blast but provided no evidence to back up that claim.
Surveillance camera video showed the explosion on a busy sidewalk, sending a child running from the flames with hands clasped over his ears.
Bodies and blood trails could be seen spread across the ground in photographs taken during the immediate aftermath.
The Britain-based Syrian Observatory for Human Rights said at least 19 people were wounded or killed. The Pentagon had not released the identities of those killed by Wednesday night.
Speaking at the State Department several hours after initial casualty reports appeared, Vice President Mike Pence did not mention the incident but hailed Trump’s leadership in combating the militants in Syria.
‘‘We are bringing our troops home,’’ Pence said in an address to more than 180 US ambassadors and chiefs of missions abroad gathered for a conference. ‘‘The caliphate has crumbled, and ISIS has been defeated.’’
In a statement issued by his office later in the day, Pence offered sympathy to the families of the Americans who were killed, condemned the attack, and said the United States would ‘‘never allow the remnants of ISIS to reestablish their evil and murderous caliphate — not now, not ever.’’
The dissonance between the vice president’s initial statement and the bloodshed on the ground in Syria reflects conflicting internal assessments about where the campaign against the Islamic State stands.
Wednesday’s attack prompted calls from Republicans and Democrats in Congress for Trump to reconsider his decision to withdraw troops.
Senator Lindsey Graham, Republican of South Carolina and a prominent Trump ally who has criticized the Syria withdrawal plan, suggested the president’s stance emboldened Islamic State fighters and encouraged such attacks.
Trump’s statements “set in motion enthusiasm by the enemy we’re fighting” and “make people we’re trying to help wonder about us, and as they get bolder, the people we’re trying to help are going to get more uncertain,” Graham said at a Senate Judiciary Committee hearing. “I saw this in Iraq. And I’m now seeing it in Syria.”
In December, Trump announced that he wanted to withdraw the US forces in as little as 30 days. That decision, over the advice and then objections of his top national security aides, was followed a day later by the resignation of Defense Secretary Jim Mattis.
Mattis signed the formal order to begin the military withdrawal before he left the Pentagon at the year’s end.
But last week, John R. Bolton, the White House national security adviser, outlined conditions for the withdrawal that could leave US forces in Syria for months or even years.
So far, the military has begun withdrawing some equipment, but not yet troops, from Syria.
The overall plan for the withdrawal has yet to be described in detail.
American military officials have offered a cautionary note that there was also a surge in violent attacks in Iraq in 2011 as US troops were withdrawing from that conflict. Wednesday’s attack in Syria, the officials said, could be viewed as a signal from the Islamic State that, contrary to Trump’s assertions that the caliphate has been destroyed, it remains a threat.
“The battle against ISIS is far from over,” said Charles Lister, a counterterrorism specialist at the Middle East Institute. “Not only are the US and their local partners still engaged in open warfare against ISIS in eastern Syria, but there have also been clear signs for many months that ISIS maintains the ability to conduct a low-level guerrilla-style insurgency in Syria, as typified by today’s attack.”
Manbij, a city in northern Syria, has been ruled by nearly all sides that are fighting in the country’s civil war, which broke out in 2011. A US-backed militia of Kurdish and Arab fighters ousted the Islamic State fighters from the city in mid-2016.
Since then, Manbij largely has been governed and protected by US-backed local councils. While the city is hundreds of miles from any territory held by the Islamic State forces, it is next to territory controlled by Turkey and its Syria rebel allies.
American forces maintain a number of bases near Manbij and run frequent patrols. And US troops have been more visible in Manbij than they have in other areas, flying US flags as part of their stabilization effort there.
Before Wednesday’s attack, two US troops had been killed in Syria since the first contingent of Special Operation troops entered the country in 2015.
Last March, Sergeant. Jonathan J. Dunbar, an elite Army commando, was killed by a roadside bomb near Manbij; in 2016, Chief Petty Officer Scott Cooper Dayton, a bomb disposal technician, was killed in a roadside blast near the town of Ayn Issa.
Material from The New York Times was used in this report.