Islamic State fighters launched a renewed assault on the Syrian city of Kobani on Wednesday night, and at least 21 people were killed in riots in neighbouring Turkey where Kurds rose up against the government for doing nothing to protect their kin.

Heavily outgunned defenders said Islamic State militants had pushed into two districts of the mainly Kurdish border city late on Wednesday, despite U.S.-led air strikes that the Pentagon acknowledged would probably not be enough to safeguard the town.

In Turkey, street battles raged between Kurdish protesters and police across the mainly Kurdish southeast, in Istanbul and in Ankara, as fallout from war in Syria and Iraq threatened to unravel the NATO member's own delicate Kurdish peace process. The street violence was the worst Turkey had seen in years.

Washington said on Wednesday night that the U.S. military and partner nations carried out eight air strikes against Islamic State fighters near the embattled Syrian city of Kobani, which it said was still under the control of Kurdish militia.

"U.S. Central Command continues to monitor the situation in Kobani closely. Indications are that Kurdish militia there continue to control most of the city and are holding out against ISIL," the Pentagon statement said, using another acronym for Islamic State.

Central Command added that the strikes, in which Jordan took part, destroyed several Islamic State targets, including five armed vehicles, a supply depot, a command-and-control compound, a logistics compound and eight occupied barracks. The United States also launched three strikes against Islamic State in Iraq.

But Kobani remained under intense bombardment from Islamic State emplacements, within sight of Turkish tanks at the nearby frontier that have so far done nothing to help.

"Tonight, (Islamic State) has entered two districts with heavy weapons including tanks. Civilians may have died because there are very intense clashes," Asya Abdullah, co-chair of the Democratic Union Party (PYD), the main Kurdish group defending the area, told Reuters from inside the town.

U.S. impatience

U.S. officials were quoted voicing impatience with the Turks, Washington's most powerful ally in the area, for refusing to join the coalition against Islamic State fighters who have seized wide areas of Syria and Iraq.

"There's no question the U.S. government thinks Turkey can do more, should do more, and that they are using excuses not to do more," said a U.S. official, who spoke on condition of anonymity. "We have been sending that message very clearly behind the scenes."

Analysts and U.S. officials said Turkey's hesitance to commit its military, NATO's second-largest, to save Kobani reflected a fear of emboldening and empowering its own Kurdish population, which has long sought greater autonomy.

Turkey says it could join only if Washington agrees to use force against Syrian President Bashar al-Assad and the Sunni Muslim jihadists fighting him in a three-year-old civil war.

Turkey's Kurds, who make up the majority in the southeast of the country, say President Tayyip Erdogan is stalling while their brethren are killed in Kobani.

Police fired tear gas and water cannon to disperse demonstrators who burned cars and tyres. Authorities imposed curfews in at least five provinces, the first time such measures have been used widely since the early 1990s.

Turkish Prime Minister Ahmet Davutoglu told reporters in Ankara that 19 people were killed and 145 wounded in riots across Turkey, vowing that Turkey's own peace process with Kurdish separatists would not be wrecked by "vandalism". Dogan news agency later said the death toll had climbed to 21.

At least 10 people died in clashes in Diyarbakir, the largest Kurdish city in Turkey's southeast. An all-day curfew there from Tuesday night was extended for another day on Wednesday. Pockets of protesters defying the curfew clashed with security forces there on Wednesday.

Others died in clashes between protesters and police in the eastern provinces of Mus, Siirt and Batman. Thirty people were wounded in Istanbul, including eight police officers.

Disturbances spread to other countries with Kurdish and Turkish populations. Police in Germany said 14 people were hurt in clashes there between Kurds and radical Islamists.

The unrest in Turkey exposes the difficulty Washington has faced in building a coalition to fight Islamic State in Syria and Iraq, two countries with complex, multi-sided civil wars in which every country in the region has a stake.

Black flag

Islamic State fighters besieging Kobani hoisted their black flag on the eastern edge of the town on Monday. Since then, U.S.-led air strikes have been redoubled. The town's defenders said earlier on Wednesday the insurgents had been pushed back, but the fighters appeared to be advancing later in the day.

Intense gunfire and loud explosions could be heard on Wednesday morning from across the Turkish frontier. Huge plumes of grey smoke and dust rose above the town, where the United Nations says only a few hundred inhabitants remain.

U.S. officials, acknowledging it will be hard to shield Kobani from the air, have played down its strategic importance.

"Air strikes alone are not going to do this. They're not going to fix this. They're not going to save the town of Kobani. We know that," Rear Admiral John Kirby, a Pentagon spokesman, told a news briefing.

Secretary of State John Kerry said: "As horrific as it is to watch in real time what is happening in Kobani ... you have to step back and understand the strategic objective."

Islamic State has been advancing on the town from three sides and pounding it with artillery despite dogged resistance from heavily outgunned Kurdish forces.

Kurdish media said Kurdish fighters thwarted a car bomb on positions in Kobani, saying the vehicle blew up before reaching its target. An Islamic State source on Twitter said the attack destroyed a police station. Neither account could be verified, but a huge explosion could be seen from across the border.

In Turkey, parliament voted last week to authorise cross-border intervention, but Erdogan and his government have so far held back, saying they will join military action only as part of an alliance that also confronts Assad.

Erdogan wants the alliance to enforce a "no-fly zone" to prevent Assad's air force flying over Syrian territory near the Turkish border and create a safe area for an estimated 1.2 million Syrian refugees in Turkey to return.

France said it supported the idea of a safe area, and Britain said it was studying it. But it is clear the proposal has not taken hold in Washington, which has been bombing Islamic State targets in Syria without Assad raising objections, and does not want to be dragged into a conflict against Damascus.

"At the moment, the American air force is flying all over Syria with the permission of the Assad government," said Tim Ripley, a defence expert for Jane's Defence Weekly.

"To try and impose a no-fly zone would potentially involve a major air war against one of the biggest air forces in the Middle East ... which would only be a distraction from the fight against (Islamic State)," he said.

Kerry, repeating lukewarm views of other U.S. officials, said: "The buffer zone is an idea that has been out there. It is worth examining, it's worth looking at very, very closely."

Pentagon spokesman Kirby said, "It is now not on the table as a military option that we are considering."