IS suicide bomber targets Kurdish forces in Syria's Raqqa amid uncertainty over US commitment

The attacked opened fire at the entrance to the Kurdish People's Protection Units (YPG) before detonating his bomb vest, the Syrian Observatory for Human Rights reported.

"A suicide attacker wearing an explosives belt blew himself up inside a YPG centre after opening fire on a security checkpoint at its entrance," Observatory chief Rami Abdel Rahman said. 

At least four civilians and a YPG fighter were killed in the attack, he added.

The Syrian Democratic Forces (SDF) battling to expel IS from its last stronghold in eastern Syria.

IS claimed responsibility for the attack they said targeted a "recruitment centre" for Kurdish forces.

"The attacker aimed at them with a machine gun then blew up his explosives vest in the middle of them," it said in a statement on the Telegram messaging app.

The YPG make up the bulk of the Syrian Democractic Forces, a Kurdish-Arab alliance backed by the US-led coalition to take on the Islamic State group. The attack comes as the US is sending mixed signals over its long-term commitment to the fight against IS, following Donald Trump's shock announcement of withdrawal from the country.

The SDF have advanced on the last IS fighters holed up in eastern Syria, near the Iraqi border, where they control a handful of villages in the Euphrates River Valley, including Sousa and Baghouz.

They also retain a presence in the country's vast Badia desert.

IS swept across large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" across areas under its control. But it has since lost most of that territory to various offensives.

The SDF ousted IS fighters from Raqqa in 2017, after a long and bloody battle against the group in their de-facto "capital".

Syria's war has killed at least 500,000 people and displaced millions more since it erupted in 2011 when anti-government protests were brutally suppressed.