Syrian Kurdish forces advanced towards an Islamic State stronghold in northeastern Syria on Sunday in an offensive backed by U.S.-led air strikes, a Kurdish official and a monitoring group said.

The Kurdish YPG militia, which last month flushed Islamic State out of the town of Kobani with the help of U.S. and allied air support, launched the offensive in the northeastern Hasaka province overnight.

It is the latest example of coordination between the U.S.-led coalition and the well-organized Syrian Kurdish militia that has been one of Islamic State's toughest enemies in Syria.

The northeastern corner of Syria is strategically important in the fight against Islamic State because it borders areas controlled by the group in Iraq.

Kurdish peshmerga forces in Iraq shelled Islamic State positions across the border in Syria in coordination with the YPG, according to a source on the frontline in the Sinjar area, where the YPG is also active.

Islamic State last year declared a cross-border "caliphate" in Syria and Iraq.

The Islamist militant group has shown signs of strain in Syria since it was driven out of Kobani. Syrian government forces have also made gains in the provinces of Hasaka and Deir al-Zor recently.

Kurdish official Nasir Haj Mansour said the Kurds had decided to launch the offensive after Islamic State reinforced its positions in the area with foreign fighters.

The aim is to take Tel Hamis, a town some 35 km (22 miles) southeast of the city of Qamishli. He said Tel Hamis was an Islamic State stronghold.

Kurdish forces had advanced to within 5 km (3 miles) of Tel Hamis, he added. "Twenty-three farms and villages - big and small - have been liberated," Mansour told Reuters by telephone.

The Kurdish fighters were giving the coordinates of Islamic State targets to the U.S.-led coalition - the same method used to call in air strikes in the battle for Kobani, he said.

The Syrian Observatory for Human Rights, which monitors the conflict, said at least 12 Islamic State fighters were killed in the fighting. Mansour said 20 Islamic State fighters had been killed.

Since driving Islamic State from Kobani, the Kurdish forces backed by other Syrian armed groups have pursued Islamic State fighters as far their provincial stronghold of Raqqa.

Hasaka province in the northeast is one of three areas where the Syrian Kurds have set up their own government since Syria descended into war in 2011.

