Syrian Kurdish forces advanced against Islamic State fighters in two separate attacks in northeastern Syria near the Iraqi border on Sunday, an organisation tracking the war reported, compounding recent losses for the militant group in Syria.

The Kurdish YPG militia has been one of Islamic State's toughest enemies in Syria and last month flushed the group out of the town of Kobani with the help of US and allied air support and Iraqi Kurdish reinforcements on the ground.

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

In an attack backed by US-led air strikes, the YPG advanced to within 5 km (3 miles) of Tel Hamis, an Islamic State-controlled town 35 km (22 miles) southeast of the city of Qamishli, Kurdish official Nasir Haj Mansour said.

The YPG had decided to launch the attack after Islamic State reinforced its positions in the area with foreign fighters.

It is the latest example of coordination between the US-led alliance and the YPG in the fight against Islamic State, which has seized wide areas of Syria and Iraq and declared them part of a cross-border "caliphate".

"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 US-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 and confirmed the advance by the YPG. Mansour said 20 Islamic State fighters had been killed.

In the second advance in the northeast, Kurdish fighters took two villages from Islamic State at the Iraqi border, helped by heavy shelling by Iraqi Kurdish peshmerga forces on the other side of the frontier, the Observatory said. It said the shelling killed eight people, five of them children.

A source on the Iraqi side of the front line in the Sinjar area said the Kurdish peshmerga forces in Iraq had shelled Islamic State positions across the border in Syria in coordination with the YPG.

The hardline 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.

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.