IS fighters seized part of a strategic hill overlooking the town of Kobane late on Saturday, a monitor said, but their progress was slowed by new strikes from the coalition of Washington and Arab allies.

A Syrian Kurdish official said IS fighters were just one kilometre from Kobane and air strikes are not enough on their own to stop them.

The dusty border town has become a crucial battleground in the international fight against IS, which sparked further outrage this weekend with the release of a video showing the beheading of Briton Alan Henning.

The video -- the latest in a series of on-camera beheadings of Western hostages -- included a threat to another hostage, US aid worker Peter Kassig, whose parents made an impassioned plea for his release.

Fighting raged around Kobane as the jihadists pressed their nearly three-week siege of the town, which saw them make some progress late Saturday, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.

"IS succeeded on Saturday night in taking the southern part of the Mishtenur hill," the Observatory's Rami Abdel Rahman told AFP.

If the jihadists seize the hilltop, he said, "the whole town of Kobane will be in their sights and it will be easier to take".

Abdel Rahman said seven new coalition strikes against IS positions were carried out in the area late Saturday and the air raids were hindering the jihadist advance.

In a statement, US Central Command said the US military carried out three air strikes in Syria on Saturday, while fighter jets, bombers and helicopters were used in six assaults against IS positions in Iraq on Sunday.

- Villages evacuated -

The battle for Kobane continued on Sunday, with shelling echoing from the town -- also known as Ain al-Arab -- and warplanes roaring overhead, an AFP reporter just across the border in Turkey said.

Standing on the roof of a building overlooking the town, 55-year-old Turkish Kurd Mahmut Yildirim said the fight for Kobane was a "do or die battle".

"This is tearing our hearts out. We cannot even get a bag of bread to our comrades fighting over there," he said.

A mortar round hit a house Sunday on Turkish territory just a few kilometres (miles) from Kobane, wounding five people, medical sources said.

The source of the fire was unclear, but as a security precaution Turkish forces ordered the evacuation of residents from two small border villages.

The number of dead in fighting on Sunday was not known, but the Observatory, which relies on a network of local sources, said at least 33 IS fighters and 23 of the town's Kurdish defenders were killed on Saturday.

IS began its advance on Kobane on September 16, seeking to cement its grip over a long stretch of the Syria-Turkey border.

The offensive prompted a mass exodus from the town and surrounding countryside, with some 186,000 fleeing into Turkey.

Extremist Sunni Muslim group IS has seized large parts of Syria and Iraq, declaring a "caliphate" in June and imposing its harsh interpretation of Islamic law.

The group has been accused of carrying out widespread atrocities including attacks on civilians, mass executions, abductions, torture and forcing women into slavery.

It has also released videos of the on-camera beheadings of two US journalists, a British aid worker and on Friday of Henning, a 47-year-old British volunteer driver who went to Syria with a Muslim charity.

Kassig, the US aid worker, was shown alive in the video and threatened by a knife-wielding militant.

- Secret talks in Turkey -

In a short video posted online on Saturday, Kassig's parents pleaded for his safe release.

"We implore his captors to show mercy and use their power to let our son go," Ed Kassig said, referring to his son, who converted to Islam, by his adopted name of Abdul Rahman.

After first launching strikes against IS in Iraq in August, Washington has built a coalition of allies to wage an air campaign against the group.

Britain and France have joined the strikes in Iraq and five Arab nations -- Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates -- have taken part in the Syria raids.

Turkey's parliament last week authorised the government to join the campaign, but so far no plans for military action have been announced.

Turkish media reported Sunday that the leader of the main Syrian Kurdish political party -- Democratic Union Party (PYD) chief Salih Muslim -- was in Turkey for secret talks with intelligence officials.

Reports said Muslim was told the PYD should distance itself from the Kurdistan Workers Party (PKK), which waged a deadly insurgency in Turkey for the past three decades, and offered the prospect of logistical support for the fight against IS.

Elsewhere, opposition fighters including Al-Qaeda affiliate Al-Nusra Front and other rebels seized a strategic plateau Syria's southern Daraa province, the highest in the province and just 12 kilometres (7 miles) from the Golan Heights.