LONDON (AFP) - Militants mounted brutal offensives on both the Syrian battlefield and in their online propaganda war on Friday, murdering a British hostage and attempting to storm a key border town.

In a video released by the Islamic State in Iraq and Syria (ISIS) group in response to US-led air strikes against it, a masked ISIS fighter beheads British driver Alan Henning and threatens a US captive, aid worker Peter Kassig.

The footage, almost identical to three previous execution films released by the group, inspired revulsion around the world and drew a stark warning from Britain's Prime Minister David Cameron.

"We will do all we can to hunt down these murderers and bring them to justice," he said, declaring that the apparent murder "shows just how barbaric and repulsive these terrorists are."

US President Barack Obama condemned the "brutal murder" and warned that the US-led coalition "will continue taking decisive action to degrade and ultimately destroy" ISIS.

The UN Security Council said the video serves as a "tragic reminder of the increasing dangers volunteer humanitarian personnel face every day in Syria."

- Knife-wielding militant -

The ISIS video, found online by US-based private monitor SITE, opens with a news report about the British parliament's vote last week to authorize air strikes against ISIS targets in Iraq. Then it cuts to Henning, on his knees against a desert backdrop and wearing an orange prison-style outfit, with a masked militant standing over him wielding a combat knife.

Henning, a 47-year-old volunteer driver who went to Syria with a Muslim charity, explains to the camera that, as a member of the British public, he is being made to pay for the parliamentary vote.

Then the militant, who has the same British accent as the killer in previous ISIS execution videos, directly addresses Cameron. "The blood of David Haines was on your hands, Cameron. Alan Henning will also be slaughtered, but his blood is on the hands of the British parliament," he declares, then cuts the hostage's throat.

Kassig, a former US army soldier and Iraq veteran in his twenties who returned to the Middle East to set up an aid agency, is then shown alive and threatened by the knife-wielding militant.

"Obama, you have started your aerial bombardment in Sham, which keeps on striking our people," the militant says, using the Arabic term for Syria and the Levant. "So it is only right that we continue to strike the necks of your people."

The militants have previously released videos showing the murders of American journalists James Foley and Steven Sotloff and of British aid worker David Haines.

- Huge plumes of smoke -

Henning's apparent murder came as other ISIS fighters besieged the Syrian town of Kobane on the Turkish border, where Kurdish militiamen are mounting a desperate defence. Huge plumes of smoke were seen rising from Kobane as its outnumbered defenders came under fire from ISIS fighters who have advanced to its gates despite US-led air strikes against them.

At least 60 mortar rounds rained down on the town, according to the Syrian Observatory for Human Rights, which described it as the heaviest bombardment since mid-September. It said a Chechen ISIS fighter was leading the assault on the town, which is considered a strategic prize.

The fierce fighting came a day after the Turkish government won authorisation from parliament to take military action in Syria and Iraq. "We will do whatever we can so that Kobane does not fall," Turkish Prime Minister Ahmet Davutoglu said, adding that about 186,000 refugees had come across from Syria in less than two weeks.

He called for a no-fly zone above northern Syria, saying that Damascus could otherwise "carry out strikes on the Aleppo region with all its might and send three million refugees into Anatolia."

President Bashar al-Assad's Syrian regime was quick to respond with a warning of its own, saying the Turkish stance "represents a real aggression against a member state of the United Nations."

Ankara has not said what action it might take to prevent ISIS fighters from taking the town, and with it unbroken control of a long stretch of Turkey's more than 900 kilometre border with Syria. The United States has been working to build an alliance against the militants, who have declared an Islamic "caliphate" on a tract of Iraq and Syria where they have committed widespread atrocities.

The Pentagon said aircraft from Saudi Arabia and the United Arab Emirates joined US warplanes in new bombing raids on Friday.

Coalition planes hit ISIS tanks, oil refineries and a training camp. American aircraft also conducted three air raids in Iraq, including two northeast of Fallujah. And a US Marine became the first American to die in the campaign, after falling into the sea as his aircraft lost power following take-off from the USS Makin Island.

With the coalition's air component expanding, Dutch Defence Minister Jeanine Hennis-Plasschaert said her country could send F-16 fighter bombers into action over Iraq by this weekend, a move welcomed by a "proud" White House.

Australia also decided to join combat sorties in support of Kurdish forces and the beleaguered Iraqi army and Canada's parliament is expected to vote on Monday to back a plan to send fighter jets.