Fighters are on the verge of seizing the key Syrian border town of Kobane, neighbouring Turkey has warned, prompting the UN envoy to Syria to appeal for immediate international action to prevent its fall.

Washington, whose air strikes have failed to stop Islamic State group fighters' advance on Kobane, said it was "very concerned" for Kurdish civilians still inside the town given the jihadists' track record of "terrible acts of violence" against ethnic minorities.

IS fighters launched their assault on the Kobane region three weeks ago, driving back the outnumbered and outgunned Kurdish militia defenders and forcing 200,000 mainly Kurdish refugees to flee to neighbouring Turkey.

But a Kurdish journalist inside the town told AFP there were still thousands of civilians who have refused to abandon their homes.

Washington and its Arab allies stepped up air raids on the advancing IS fighters -- AFP correspondents on the Turkish side of the border reported hearing at least eight strikes around Kobane on Tuesday.

The Pentagon said coalition strikes near Kobane on Monday and Tuesday had damaged or destroyed several armed vehicles, anti-aircraft artillery, a tank and a fighter "unit".

But they failed to stop IS fighters from penetrating eastern neighbourhoods of the town on Monday, sparking fierce street battles with the Kurdish defenders.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the Kurds had succeeded in forcing the fighters to retreat in some areas they had entered.

But Turkish President Recep Tayyip Erdogan warned that the town was "about to fall", saying a ground operation was needed to defeat the fighters.

"I am telling the West -- dropping bombs from the air will not provide a solution," he said.

Turkey last week won parliamentary approval for military intervention against IS in Syria and Iraq, but it has yet to announce any firm plans despite the advance of the fighters on its doorstep.

Kobane, also known as Ain al-Arab, would be a major prize for the fighters, as it would give them unbroken control of a long stretch of Syria's border with Turkey.

Pro-Kurdish demonstrators angered by Ankara's failure to act so far clashed with police across Turkey, leaving at least a dozen people dead and many wounded.

The Kurdish diaspora also held protests in Europe, with dozens of demonstrators bursting into the European Parliament in Brussels.

'Need to act now'

The UN envoy to Syria, Staffan de Mistura, appealed to the international community to act immediately to defend Kobane against the much better armed IS fighters, who seized large amounts of heavy weaponry from retreating troops in a lightning offensive in neighbouring Iraq in June.

"The world, all of us, will regret deeply if (IS) is able to take over a city which has defended itself with courage but is close to not being able to do so. We need to act now," he said.

"The international community needs to defend them. The international community cannot sustain another city falling under (IS)".

Iran, which unlike the West supports President Bashar al-Assad's regime, criticised the "passivity of the international community" in the face of the IS offensive.

But analysts said that a lack of ground support for air strikes against IS, and Turkey's reluctance to step in militarily, had all but sealed Kobane's fate.

"It is practically too late to save Kobane at this stage," said Mario Abou Zeid, a Beirut-based analyst with the Carnegie Middle East Centre.

The United States and its allies have launched nearly 2,000 air raids against fighters in both Iraq and Syria -- with Canada the latest to join the aerial war after lawmakers on Tuesday approved a six-month mission.

But Abou Zeid said: "The expansion of Islamic State forces in Kobane is new proof that the air strikes campaign is failing to achieve its objective."

'Fight to the last'

At least 412 people, more than half of them fighters, have been killed in and around Kobane since IS began its assault in mid-September, according to the Observatory.

The defenders have vowed to "fight to the last person," said Kobane activist Mustafa Ebdi.

Kurdish journalist Ozgur Amed said thousands of civilians were still inside the town. "They don't want to become refugees," he said.

"Our morale is still good," he told AFP by telephone from inside Kobane. "We are just afraid of the humanitarian situation getting worse."

White House spokesman Josh Earnest told reporters that Washington was "very concerned" about the safety of civilians in Kobane.

"Once again, we are seeing this extremist organisation, in the name of an otherwise peaceful religion, Islam, perpetrating terrible acts of violence against religious and ethnic minorities."

"This is something we remain concerned about."

The FBI appealed for help to track people planning to join the IS group in combat.

It said about a dozen Americans are fighting with the group, which has been accused of carrying out widespread atrocities including mass executions and torture.