Bombings suspected to have been carried out by the Islamic State group killed at least eight people in northeastern Syria hours after a top US commander visited, security forces said Sunday.

Washington regards the Kurdish-led militia that controls most of the northeast as the most effective fighting force against IS in Syria and the head of US Central Command General Joseph Votel made a secret visit Saturday to confer with US military advisers working with them.

A CENTCOM spokesman declined to give details of the visit, saying only that Votel had visited several location inside Syria on the highest-ranking visit to the country since the 2011 outbreak of the civil war.

Two IS suicide bombers struck the centre of Qamishli, a mainly Kurdish city that is the de facto capital of the swathes of northern Syria where Kurdish militia have set up a self-declared autonomous administration.

The bombers hit a restaurant and a bakery in the Christian Wusta neighbourhood of the city that is controlled by a breakaway Christian militia that backs the Damascus regime.

A militia spokesman said three Christians were killed and 15 wounded in the bombings.

The IS-affiliated Amaq news agency reported the bombings but issued no claim.

Hours earlier, two car bombs hit a Kurdish checkpoint outside the town of Tal Tamr, about 40 kilometres (25 miles) from the Turkish border.

Kurdish security force spokesman Abdallah Saadoun told AFP that there had been advance warning of the attack but that five security force personnel were killed nonetheless.

Amaq said a "suicide operation" had killed more than a dozen Kurdish security personnel but issued no claim. 

Washington has kept up its support for the Kurdish militia in northeastern Syria despite strong opposition from NATO ally Ankara whose misgivings have prevented the delivery of heavier weapons.

Ankara regards the main Syrian Kurdish party as a puppet of the Kurdistan Workers Party (PKK) which has waged a deadly three-decade insurgency in southeastern Turkey.

It has repeatedly shelled Syrian Kurdish positions when the militia has made advances in border territory that Turkish commanders consider sensitive.

Source: AFP