The black flag of Isis has been torn down from a hill overlooking the town of Kobani after it was captured by Kurdish fighters.

The symbolic move came after the US-led coalition launched a series of 21 air strikes on Isis positions in and around the besieged town.

Kurdish fighters captured the hill of Tel Shair after it was targeted by one of the coalition air strikes, a Kurdish official has said.

Isis still controls more than a third of the predominantly Kurdish town, where the battle continues to rage despite more than two weeks of air strikes by the coalition.

Isis militants last week raised two black flags at the town's eastern entrance, one over a building and another on a hill.

The US Defence Department today said the US-led coalition launched a total of 22 air strikes on Isis positions in Syria, with all but one targeting sites in and around Kobani.

"Over the past night there have been very intense air strikes by the coalition that targeted several Daesh positions in and near Kobani," said Idriss Nassan, deputy head of Kobani's foreign relations committee, using an Arabic acronym to refer to Isis (also known as Islamic State and Isil).

A black flag belonging to the Islamic State is seen near the Syrian town of Kobani on October 6 (Reuters)

Nassan said one of the air strikes targeted the Tel Shair hill, which was then captured by Kurdish fighters who brought down the black flag.

More than 500 people have been killed during the fighting in and around Kobani, mostly fighters from both sides, while more than 200,000 people have been forced to flee across the border into Turkey.

White House spokesman Josh Earnest today said the impact of the air US-led air strikes will be limited without the help of Syrian opposition fighters on the ground.

"Now, airstrikes will have an impact. But that impact is constrained by the fact that there aren't forces on the ground that can follow up on those airstrikes to end that siege," Earnest said at a press briefing.

Meanwhile, Turkish warplanes have struck suspected Kurdish rebel positions in southeastern Turkey, it was reported today.

It was the first major air strike against the Kurdistan Workers' Party, or PKK, since peace talks began two years ago to end a 30-year insurgency in Turkey.

The move added to tensions between the key US coalition partner and PKK, a militant group listed as a terrorist organization by the US that is also among the fiercest opponents of Isis.

The attack follows a week of violence in Turkey sparked by the advance of Isis on Kobani.

More than 30 people, including two police officers, were killed last week as Kurds, angered at what they said was a Turkish impediment to efforts to defend Kobani, clashed with police and supporters of an Islamist group in cities across Turkey.