BEIRUT (Reuters) - Islamic State said on Tuesday it had launched a deadly attack on Kurdish forces in northern Syria overnight, the first such claim by the group since U.S.-backed forces proclaimed the capture of its last territory in Syria last week.

A spokesman for the U.S.-backed militia that controls Manbij said a terrorist attack on a checkpoint at the city’s entrance killed seven fighters on duty. A U.S. official said all the casualties were among members of the Syrian Democratic Forces (SDF).

Islamic State said in a statement that “caliphate soldiers attacked a checkpoint of the apostate PKK (Kurdistan Worker’s Party)” west of Manbij using machine guns and killed nine fighters.

Some 30 km (20 miles) from the Turkish border, Manbij has been a flashpoint in the Syrian conflict and occupies a critical spot, near the junction of three separate blocks of territory that form the spheres of Russian, Turkish and U.S. influence.

It was seized in 2016 from Islamic State by Syrian militia allied to the SDF, a militia force spearheaded by the Kurdish YPG and which controls roughly a quarter of Syria.

The SDF on Saturday proclaimed the capture of Baghouz, Islamic State’s last territory in Syria, eliminating its rule over a self-proclaimed “caliphate”, but the jihadists remain a threat, with followers around the world. [nL8N2171Y5

“We cannot blame a specific side for this attack, but it is definitely linked to the proclamation of victory over Daesh,” Manbij Military Council spokesman Sharfan Darwish said.

“The purpose of these attacks is to incite sedition. Also the parties troubled by the defeat of Daesh are trying to threaten stability in the area,” he told Reuters.

Turkey views the YPG as an extension of the PKK, which has waged a 34-year-old insurgency in Turkey’s mainly Kurdish southeast.

Originally an offshoot of al Qaeda, Islamic State took swathes of Iraq and Syria from 2014, imposing a reign of terror with public beheadings and attacks by supporters abroad.