°®Âþµº

A U.S.-backed force and two Syrian activist groups say Islamic State militants have carried out an attack in the city of Raqqa in northern Syria.

The early Wednesday attack targeted a post of the Kurdish-led Syrian Democratic Forces in Raqqa, which was once the Islamist extremists' de facto capital.

"The reports about tonight's ISIS attack in Raqqa appear exaggerated. An SDF fighter stationed in the city told me the attack involved 1 suicide bomber & 1 person throwing a grenade," said Elizabeth Tsurkov, a Fellow at the Foreign Policy Institute on Twitter.

IS claimed responsibility for the attack through its affiliated Amaq news agency, saying a sole "suicide bomber" carried out the attack using hand grenades.

The attack comes as Turkey is expected to launch an offensive against the Kurdish fighters in northeastern Syria.

Turkish forces were seen crossing into Kurdish-held areas of Syria on Wednesday.

Read more: Syria's Kurds abandoned to a perfect storm of Turkish aggression and US isolationism

The Kurdish fighters say IS launched three suicide attacks against its positions in Raqqa. There was no word on casualties.

Raqqa is being Slaughtered Silently, an activist collective, reported an exchange of fire and a blast.

The UK-based Syrian Observatory for Human Rights, a war monitoring group, says the attack involved two IS fighters who engaged in a shootout before blowing themselves up.

Kurdish forces warned on Wednesday that the Turkish operation could lead to an IS resurgence in eastern Syria, and that they may not be able to guard IS prisoners if their fighters were engaged in combat with Turkish forces.