The US-led coalition pounded positions held by Islamic State militants in the Syrian border city of Kobane on Thursday in some of the most intensive strikes in the air campaign so far, a Kurdish official and an activist group said.

But despite the airstrikes overnight and into the morning, the IS fighters managed to capture a police station in the east of the town, said the British-based Syrian Observatory for Human Rights.

The militants now control more than a third of the strategic border town, added the group.

The fighting over Kobane has yet again brought Syria's civil war to Turkey's doorstep, and allies are pressing Ankara to take a more robust role in the fight against IS.

Turkish Foreign Minister Mevlut Cavusoglu said on Thursday that it was unrealistic to expect Turkey to launch a ground war against IS on its own.

Cavusoglu spoke at a news conference in Ankara attended by visiting NATO chief Jens Stoltenberg, who said there is no easy way to push back the siege of Kobane.

"ISIL poses a grave threat to the Iraqi people, to the Syrian people, to the wider region, and to NATO nations," Stoltenberg said, using another name for IS. "So it is important that the whole international community stays united in this long-term effort."

Cavusoglu said Turkey is prepared to take on a bigger role once a deal is reached with the coalition.

"Turkey will not hold back from carrying out its role," he added.

IS launched its offensive on Kobane in mid-September, capturing several nearby Kurdish villages and steadily tightening the noose around the city. The fighting has forced at least 200,000 city residents and villagers to flee across the frontier into Turkey.

However, Idriss Nassan, an official with Kobane's local government, denied the militants were in control of a third of the town on Thursday.

He confirmed that the police station was taken by IS, but said it was later destroyed in an airstrike. He said the Kurdish fighters managed to regain several other areas on Thursday.

"I can confirm that they don't control a third of the city, there is only a small part of Kobane under the control of Daesh," said Nassan, using the Arabic name for IS.

Both Nassan and the Observatory said more than 20 airstrikes have been conducted in the area since Wednesday afternoon.

The Observatory's chief, Rami Abdurrahman, said more than 500 people have been killed in and around Kobane since the fighting began.

IS has brought reinforcements from its stronghold in the border town of Jarablous and the town of Manbij and Aleppo province, he added.