Three additional U.S. service members were wounded, U.S. Central Command said in a statement.
The incident occurs as the Pentagon begins its drawdown from Syria in keeping with President Trump’s announcement last month that the Islamic State had been defeated and troops would be coming home.
The president’s surprise Dec. 19 announcement 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 erupting 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.
White House press secretary Sarah Sanders praised the “brave American heroes” who died in the attack. “Our service members and their families have all sacrificed so much for our country,” she said in a statement.
The White House said Trump had been “fully briefed” on the incident, the most deadly since U.S. troops arrived in Syria in 2015. Previously, two American service members had been killed in action there.
Speaking at the State Department several hours after initial casualty reports appeared, Vice President 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 U.S. ambassadors and chiefs of missions abroad gathered for a conference in Washington. “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.
Trump, announcing last month that the force of more than 2,000 U.S. service members would be leaving Syria, heralded categorical victory over the Islamic State more than four years after U.S. forces launched an international coalition to dislodge militants from their self-declared “caliphate” straddling Syria and Iraq.
The president’s declaration generated consternation from foreign partners, including France and Britain, and accusations of abandonment from a U.S.-backed Syrian Kurdish force that has suffered thousands of casualties. Defense Secretary Jim Mattis resigned the next day.
Since then, Trump and other senior officials have at times used more cautious rhetoric in addressing the Islamic State, which the Pentagon has said retains significant combat power, especially in eastern Syria, where it continues to hold territory.
Despite Trump’s initial suggestion that troops would depart immediately, the White House subsequently has said there is no timeline for the U.S. departure. Compounding the confusion, military officials say they are proceeding with orders to withdraw within about four months.
On Friday, the military announced it had begun withdrawing equipment but not forces. It’s not clear what weaponry or equipment has been removed from Manbij, which was reclaimed from militants in 2016.
Hundreds of U.S. troops have been stationed in Manbij in an attempt to prevent extremists from regaining strength and to foster stability in an area strategic to both NATO ally Turkey to the north and Syrian Kurdish forces who have been the chief U.S. partner against the Islamic State.
Turkey considers some Syrian Kurds, including U.S. partner forces, to be part of a terror group.
U.S. troops have been more visible in Manbij than they have in other areas, flying U.S. flags as part of their stabilization effort there.
Lawmakers of both parties seized on the attack as proof that Trump should rethink his Syria plans. Sen. Lindsey O. Graham (R-S.C.), who has gone between lauding and excoriating the president, made an impassioned speech at the start of William P. Barr’s confirmation hearing to serve as attorney general, imploring Trump to reconsider his position in light of the carnage.
“My concern about the statements made by President Trump is that you set in motion enthusiasm by the enemy we’re fighting. You make people we are trying to help wonder about us, and as they get bolder, the people we’re trying to help are going to get more uncertain. I saw this in Iraq, and I’m now seeing it in Syria,” Graham said.
“I know people are frustrated. But we’re never going to be safe here unless we’re willing to help people over there who will stand up against this radical ideology,” he added. “To those who lost their lives today in Syria, you were defending America in my view . . . and I hope the president will look long and hard at what we’re doing in Syria.”
Sen. Jack Reed (D-R.I.), at a meeting of HillVets, a Washington veterans group, said the United States was failing to demonstrate the global leadership it had shown in the past.
“Today’s very tragic situation is a reflection that ISIS is not gone and done with,” he said. Reed said it was vital to keep military pressure on the group and warned that ISIS leaders interpreted Trump’s calls for a withdrawal from Syria as “a great relief of the pressure on them.”
As the Pentagon begins its withdrawal, it remains unclear whether the White House plan will include an exit for several hundred troops now stationed at the Tanf garrison in southeast Syria. While national security adviser John Bolton has suggested that base, seen as key to constraining Iran’s influence in Syria, could remain open, military officials are planning to shut it down.
Zakaria Zakaria in Istanbul and Karen DeYoung, Greg Jaffe, Carol Morello, John Wagner, Karoun Demirjian and William Branigin in Washington contributed to this report. Ryan reported from Washington.