The Islamic State group on Tuesday claimed an attack on US-backed forces in the northern Syrian city of Manbij, days after their "caliphate" was declared defeated.
Manbij is a former IS stronghold that is now ruled by a military council affiliated with the Syrian Democratic Forces (SDF), the US-backed Kurdish-led alliance that declared victory over IS in its last redoubt in eastern Syria on Saturday.
At around midnight (2200 GMT) on Monday, gunmen opened fire at fighters manning a checkpoint at the entrance to the city, killing seven, the council said.
IS claimed responsibility for the attack in a statement published on its social media channels on Tuesday.
"The caliphate's soldiers attacked a checkpoint... west of Manbij city last night," it said.
A spokesman for the Manbij Military Council, Sherfan Darwish, said it could be a revenge attack.
"After the victory over IS, we have entered the phase of sleeper cells," Darwish told AFP.
"These sleeper cells are being activated and carrying out attacks but we will foil their operations."
The Syrian Observatory for Human Rights, a Britain-based monitor of the war, said it is "the first attack of its kind" since the SDF declared the defeat of the "caliphate" last week.
Observatory head Rami Abdel Rahman said it was also the bloodiest attack in Manbij since January 16, when 19 people -- including four US service personnel -- were killed in a suicide bombing claimed by IS.
IS has vowed to carry out revenge attacks against the SDF over the six-month offensive against it that culminated in the jihadists' defeat in the village of Baghouz, close to the Iraqi border, on Saturday.
The Observatory said hundreds of SDF members have been killed in attacks believed to have been carried out by IS sleeper cells since August.
Manbij is also a major point of contention between the Kurds, who lead the SDF, and neighbouring Turkey, which is deeply opposed to their autonomous administration in northeastern and parts of northern Syria.
The city is one of the few areas west of the Euphrates that remains under Kurdish influence after Turkish troops and their Syrian rebel allies overran the Kurdish enclave of Afrin in March last year.
Ankara has threatened to launch a new offensive to dislodge the People's Protection Units (YPG) -- the Kurdish force that forms the backbone of the SDF -- from the entire length of the border.
The YPG is considered a terrorist group by Ankara because of its links to the Kurdistan Workers Party (PKK), the outlawed rebel group that has fought a deadly insurgency for self-rule in southeastern Turkey since 1984.