US service members were killed in the attack, according to a tweet from the spokesperson for the US-led coalition Operation Inherent Resolve.
“U.S. service members were killed during an explosion while conducting a routine patrol in Syria today. We are still gathering information and will share additional details at a later time,” the tweet said.
The coalition has not said how many service members were killed. Prior to Wednesday’s attack, only two US service members had been killed in action in Syria since the start of the campaign in 2014.
There were several reported casualties in the city a militia controlling the city and a UK-based monitoring group said.
The ISIS-affiliated Amaq agency said the attack in the northern city of Manbij was carried out by a suicide bomber with an explosive vest.
“An explosion in Manbij’s busy market street, initial reports of casualties,” spokesman of the Manbij military council Shervan Darwish wrote on Twitter.
The UK-based Syrian Observatory for Human Rights said at least eight people were killed.
A spokesperson for Operation Inherent Resolve tweeted Wednesday that the coalition is “still gathering information” about the explosion.
White House spokeswoman Sarah Sanders said President Donald Trump has been briefed on the situation. Vice President Mike Pence has also been briefed, according to a tweet from his press secretary.
However, Pence made no mention of the attack or the deaths of US service members while making remarks at the Global Chiefs of Mission conference at the US State Department Wednesday, claiming “The caliphate has crumbled, and ISIS has been defeated.”
The attack comes less than a month after Trump announced that US troops would withdraw from Syria. In making his announcement, Trump declared in a video released on Twitter: “We have won against ISIS. We’ve beaten them and we’ve beaten them badly. We’ve taken back the land and now it’s time for our troops to come back home.”
The US has about 2,000 troops in Syria, with no specific date for their withdrawal. Last week, the US began withdrawing some military ground equipment from Syria, according to an administration official with direct knowledge of the operation.
During his surprise visit to Iraq on December 26, Trump was warned by military commanders that — despite his claims — ISIS was not entirely defeated in Syria. People familiar with the President’s reaction said the conversation was eye-opening for a leader who, days earlier, claimed the terror group was defeated “badly” in the country.
The discussion occurred inside a tan tent at the al-Asad airbase west of Baghdad and included the US Ambassador to Iraq Douglas Silliman, Lieutenant General Paul LaCamera, national security adviser John Bolton, and the first lady Melania Trump, along with other officials.
Trump was told that pockets of ISIS militants remained in the Euphrates River valley and that the US military had not yet eliminated all of their strongholds. Commanders told him the US had been successful in taking back other areas but that the job was not finished.
The people familiar with the conversation described it as sobering, and said it broke through to Trump in a way his conversations with national security officials in Washington had not. Coming days after Trump’s decision to withdraw troops from Syria, it centered partly on the remaining challenges of going after ISIS fighters in pockets of Syria.
In addition to the briefing from the commanders, Trump found the massive security apparatus on the trip surprising — something his advisers told him was reflective of the remaining challenges against ISIS.
CNN’s Eyad Kurdi and Mohammed Tawfeeq, Jennifer Hansler and Paul Murphy contributed to this report.