Kurdish protesters throw stones as they clash with riot police (foreground) in Diyarbaki, Turkey.  REUTERS/Sertac Kayar

At least 21 people have been killed in street violence in Turkey as the Kurdish population protested against the government for its inaction against Isis militants.

The militants had pushed into two districts of the Syrian border town of Kobani on Wednesday night within sight of Turkish tanks at the frontier, reports Reuters.

Kobani remained under intense bombardment by the Islamic State militants despite US-led air strikes.

The US and its allies carried out eight air strikes against Islamic State fighters near the city, destroying five armed vehicles, a supply depot, a command-and-control compound, a logistics compound and eight occupied barracks.

Air strikes were also carried out against Isis in Iraq.

US officials have expressed disappointment with Turkey, considered Washington's most powerful Nato ally in the area, for refusing to join the coalition against Isis.

Turkey's reluctance to commit its military, Nato's second-largest, to save Kobani is believed to stem from a fear of empowering its own Kurdish population, which has been seeking greater autonomy.

Turkey has said it could join only if Washington agrees to use force against Syrian President Bashar al-Assad and the Sunni Muslim jihadists involved in a three-year-old civil war. It has sought a buffer zone in Syria along the border to ensure security and ease the refugee influx.

The US seemed to be at odds with its allies over this. France was all for it, while the UK was said to be exploring the idea.

Meanwhile, the US chose to downplay the importance of Kobani, preferring to focus on degrading the capability of Isis to sustain itself.

"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.

But experts say control over the town would see the militants rule over a stretch of territory between Raqqa, Syria and Turkey covering more than 100kms.

The strategic corridor along the Turkish border will link the group's positions in Aleppo to the west and Raqqa to the east, reports CNN.