Jihadists fighting to take the strategic Syrian border town of Kobane advanced Wednesday despite intensified US-led air strikes, as deadly protests over the fate of its Kurdish residents shook neighbouring Turkey.

As pressure grows for international action to halt the advance of the Islamic State (IS) group's fighters, France threw its weight behind calls for a buffer zone on the Syrian-Turkish frontier.

The top US and British diplomats said they were willing to "examine" the idea of a safe haven, but the White House later denied it was considering such a move.

And the Pentagon said air strikes alone were not enough to prevent Kobane from falling.

Ultimately, "capable" ground forces -- rebels in Syria and Iraqi government troops -- would have to defeat IS, spokesman Rear Admiral John Kirby said.

Kobane has become a symbol of resistance against IS, which proclaimed an Islamic "caliphate" across swathes of Iraq and Syria, carrying out beheadings and other atrocities.

Demonstrations in Turkey over Ankara's lack of action to support Kobane's predominantly Kurdish residents have triggered clashes in which at least 19 people were killed.

For the first time in more than two decades, a curfew was declared in six Turkish provinces after the unrest.

In Germany, police used batons, pepper spray and water cannon as Kurds and Yazidis clashed with radical Muslims in two northern cities in violence that injured at least 23 people.

The three-week IS group assault on Kobane has sent some 200,000 people flooding across the border into Turkey, and some residents said hundreds more remained two days after jihadists breached the town's defences.

"There are 1,000 civilians who refuse to leave," said activist Mustafa Ebdi.

"One of them, aged 65, said to me: "Where would we go? Dying here is better than dying on the road.'"

US and coalition aircraft targeted IS fighters near the town Wednesday, launching six attacks to help the defenders, the US military said.

The strikes destroyed an armoured personnel carrier, artillery and several vehicles, Central Command said.

The sounds of heavy gunfire and mortar shells were heard from the Turkish side of the border, an AFP reporter said, as fierce street battles raged.

"The raids helped prevent the fall of the town, but what is needed now is weapons," said Ebdi.

An IS fighter carried out a suicide truck bombing in east Kobane, but there was no immediate news of casualties, the Syrian Observatory for Human Rights said.

Observatory directory Rami Abdel Rahman said IS forces had advanced around 100 metres (yards) towards the town centre Wednesday evening, but that fighting had subsided slightly.

But he added that IS group reinforcements were heading from Syria's Raqa province.

The Observatory says about 400 people, more than half of them jihadists, have been killed in and around Kobane since the assault began in mid-September.

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

France said it supported a proposal by Ankara to create a safe zone along its frontier with Syria to ensure security and to host fleeing refugees.

In a telephone conversation with his Turkish counterpart, Recep Tayyip Erdogan, President Francois Hollande "gave his support to the idea... of creating a buffer zone between Syria and Turkey to host and protect displaced people," the French presidency said.

In Washington, US Secretary of State John Kerry told reporters: "The buffer zone is an idea that's out there. It's worth examining; it's worth looking at very, very closely."

And British Foreign Secretary Philip Hammond said after meeting Kerry: "We are at the stage of exploring this."

But White House spokesman Josh Earnest said "it's not something that is under consideration right now."

Ankara has come under increasing pressure to act in Kobane but its response has been complicated by concerns over emboldening Kurdish separatists, who have waged a deadly insurgency in Turkey for the past three decades.

Turkey has detained dozens of Kurds who crossed the border from Kobane on suspicion of having links with the outlawed Kurdistan Workers' Party (PKK), officials said Wednesday.

The United States, along with Arab, European and other allies have launched nearly 2,000 air raids against jihadists in Iraq and Syria.

US President Barack Obama was due to meet military chiefs later Wednesday to discuss the battle.

More than 180,000 people have been killed in Syria since an uprising against President Bashar al-Assad's regime began in 2011, morphing into a several-sided civil war that has drawn thousands of jihadists from overseas.