Last week, the USA began withdrawing some military ground equipment from Syria, according to an administration official with direct knowledge of the operation. The town is controlled by a militia allied to US-backed Kurdish forces.
A USA official who declined to be named said four American troops had been killed and three wounded in the blast on Wednesday, which an Islamic State-affiliated website said was the work of a suicide bomber.
The attack comes as the USA has begun the process of withdrawing its troops from the country, which President Donald Trump ordered last month, surprising US officials and allies and prompting Defense Secretary Jim Mattis to resign.
The US military has confirmed servicemen have been killed while on routine patrol in northern Syria.
US-backed SDF's operations played part in the removal of ISIL fighters from vast majority of the territories it captured Syria and Iraq since 2014 and Washington has defended its alliance with the group through these years.
The names of the American victims are being withheld until their families can be notified.
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"You set in motion enthusiasm by the enemy we're fighting".
The Washington Post suggests that the deaths cast doubt on the wisdom of President Trump's decision to withdraw American forces from Syria. Why? Trump initially appeared to want to withdraw from Syria rapidly, but has said since the withdrawal will be carried out more gradually.
Manbij has been held by USA -backed fighters allied to the Kurdish YPG militia since they took it from Islamic State in 2016. Syrian army troops entered the area soon after.
"I want to take a moment to express my deepest condolences to the families of the fearless American heroes who laid down their lives yesterday in selfless service to our nation", Trump said in his first public comments about Wednesday's attack. "We are still gathering information and will share additional details at a later time".
The soldiers were killed during an explosion in the northern town of Manbij.
"An explosion hit near a restaurant, targeting the Americans, and there were some forces from the Manbij Military Council with them", one said.
Turkey has long demanded the SDF fighters to move to the east of the Euphrates River in Syria and leave the control of the town of Manbij to the Turkish army and Turkey-backed opposition forces. Among the four who died were two soldiers and two civilians.
One of the witnesses said there was a "heavy" presence of military aircraft over Manbij following the blast, which took place near a vegetable market.