Kurdish fighters battled the Islamic State group in northern Syria on Tuesday, after President Barack Obama said the US-led coalition was intensifying air strikes against the jihadists in the conflict-riven country.

The Kurds are trying to repel a major offensive which the extremist group launched Monday against villages in the northern provinces of Raqa and Hasakeh.

The Syrian Observatory for Human Rights said about 80 IS fighters had been killed since Sunday morning in the fighting and US-led air strikes.

Obama said Monday night the coalition fighting the jihadist group -- also known as ISIL -- would step up its campaign in Syria, while cautioning a long battle remained.

"We're intensifying our efforts against ISIL's base in Syria. Our air strikes will continue to target the oil and gas facilities that fund so much of their operations," he said.

"This is a long-term campaign," the US president said.

Obama said more than 5,000 air strikes had been carried out against the group, eliminating "thousands of fighters, including senior ISIL commanders".

In recent days, the coalition has bombarded IS in a series of heavy raids, particularly targeting its de facto Syrian capital Raqa.

US Defence Secretary Ashton Carter said the raids were intended to help Kurdish forces, who have been a leading ground force partner for the American-led coalition in Syria.

- IS fightback -

In January, Kurds secured the symbolic town of Kobane on the border with Turkey after four months of IS attempts to overrun it.

And in recent weeks, they recaptured the key town of Tal Abyad, depriving IS of a conduit through which it transported weapons and fighters.

But IS has fought back with its offensive in parts of Raqa and Hasakeh provinces.

The Observatory said the jihadists took the town of Ain Issa, 55 kilometres (35 miles) from Raqa, but Kurdish officials and activists said the extremists had been repelled.

The Kurds also reclaimed more than 10 villages in Raqa and Hasakeh that were briefly overrun during the IS offensive, the Observatory said.

"The coalition aircraft have played an effective role in the recapture," Observatory director Rami Abdel Rahman said, adding heavy fighting continued in the two provinces.

- Priest feared kidnapped -

Elsewhere, the Observatory said six children were among at least nine people killed in regime strikes on the Naseeb area in the southern province of Daraa.

And dozens of families fled the IS-held town of Palmyra after regime planes launched some 90 air raids in less than 48 hours.

The Observatory said the aerial bombardment, which killed five people but did not affect Palmyra's historic ruins, was the most intense against the town since IS captured it on May 21.

The Franciscan Custody of the Holy Land, meanwhile, said it feared one of its priests had been kidnapped, possibly by the Al-Qaeda-affiliated Al-Nusra Front, in the northwestern province of Idlib.

It lost contact with Father Dhiya Aziz, from the Christian village of Yacoubieh, on Saturday.

"Some militants of an unknown armed brigade, perhaps connected with (Al-Nusra), came to take him away for a brief interview with the emir (leader) of the place.

"From that moment we do not have any more news and we are unable to trace his whereabouts at the present moment. We are doing everything possible to locate the place of his detention and secure his release."

Christians once made up five percent of Syria's population, but many have fled since the conflict began with anti-government protests in March 2011, particularly with the rising presence of jihadists.

Most of Idlib province is now held by a coalition of rebels including Al-Nusra.

On Tuesday, the Observatory said regime forces released a group of prisoners in the province in exchange for 26 people being held by rebels, most of them women and children.

In Damascus, state media said parliament had approved a deal under which Tehran will extend Syria a $1-billion-credit line, the third which the key ally has offered since the war broke out.