Turkey is to allow Iraqi Kurdish fighters to reinforce fellow Kurds in the Syrian town of Kobani, while America air-dropped arms for the first time to help the defenders resist an Islamic State assault while US jets also carried out a series of air strikes near the town.

US Secretary of State John Kerry said Washington had asked Ankara to let Iraqi Kurds cross its territory so they could help defend the town, which lies on the Turkish frontier. Mr Kerry said he hoped the Kurds would "take this fight on".

Kurdish militias in Kobani have been fighting off an Islamic State (IS) offensive since September without, until now, outside help apart from US-led airstrikes on the jihadists. The town, which is besieged by IS on three sides, lies on the frontline of the battle to foil the radical group's attempt to reshape the Middle East.

However, Ankara views the Syrian Kurds with deep suspicion because of their ties to the PKK (Kurdistan Workers' Party), a group that waged a decades-long militant campaign for Kurdish rights in Turkey and which Washington regards as a terrorist organisation.

Speaking in Indonesia, Mr Kerry acknowledged Turkish concerns about support for the Kurds, and said the airdrop of supplies provided by the Kurdish authorities in Iraq did not amount to a change of US policy.

He indicated the battle against IS, a group also known as ISIL and which has seized large areas of Syria and Iraq, was an overriding consideration.

"We understand fully the fundamentals of (Ankara's) opposition and ours to any kind of terrorist group, and particularly, obviously, the challenges they face with respect to the PKK," he said.

But he added: "We cannot take our eye off the prize here. It would be irresponsible of us, as well as morally very difficult, to turn your back on a community fighting ISIL."

Iraqi Kurdish official Hemin Hawrami said 21 tonnes of weapons and ammunition supplied by the Iraqi Kurds had been dropped in the small hours of yesterday.

The US military also conducted six air strikes against IS militants near Kobani on Sunday and yesterday, the US Central Command said.

Mr Kerry said he and President Barack Obama had spoken to Turkish authorities before the air drops "to make it very, very clear this is not a shift of policy by the United States".

"It is a crisis moment, an emergency where we clearly do not want to see Kobani become a horrible example of the unwillingness of people to be able to help those who are fighting ISIL," he added.

Turkey has stationed tanks on hills overlooking Kobani but has refused to help the Kurdish militias on the ground, suspicious of their links to the PKK and demanding broader US action that would target Syrian President Bashar al-Assad, as well as Islamic State.

Turkish Foreign Minister Mevlut Cavusoglu said Ankara was facilitating the passage of Iraqi Kurdish peshmerga forces, which have also fought IS when the militants attacked the Kurds' autonomous region in Iraq over the summer. Syrian Kurdish officials, however, said no backup had arrived.

Mr Cavusoglu stopped short of saying whether Ankara backed the US decision to air-drop the weapons.

Turkey's refusal to intervene in the battle against IS has led to growing frustration in America. It has also provoked lethal riots in southeastern Turkey by Kurds furious at Ankara's failure to help Kobani or at least open a land corridor for volunteer fighters and reinforcements to go there.