Islamic State militants may be using US weapons intended for Kurds defending the Syrian town of Kobane, according to the US defence department.

The US dropped 27 bundles of small arms, ammunition and other weapons on Kobane. However, an Islamic State video seems to show that some of the material ended up in its hands and is now being used against the Kurds.

The border town has been under assault for several weeks, with most of its population forced to leave. Turkey has only recently agreed to let members of its Kurdish population cross the border to help its Syrian defenders and the US has been supporting them with air strikes.

A statement after the air drop on Monday said one bundle of weapons went astray but was destroyed to prevent it falling into enemy hands, and that all the others were safely delivered.

Pentagon spokesman Rear Adm John Kirby said he could not confirm that the video was authentic. However, he added: "They are certainly of the kinds of material that was dropped ... so it's not out of the realm of the possible in that regard.

"When we have something definitive that we can provide in terms of an assessment on that we'll do that."

Islamic State has made sweeping territorial gains in the last few weeks, helped by the weakness of Iraqi military forces and the anarchy in Syria. In Iraq, US and other Western powers are operating at the request of the government in support of its own troops, though in Syria President Bashar al-Assad has not given his permission.

A UN aid convoy carrying lifesaving supplies for Syrian children fleeing the fighting in Kobane managed to cross over the battle line in Aleppo for the first time in months, a spokesman for the United Nations Children's Fund (UNICEF) said yesterday.

Christophe Boulierac said that five trucks departing from the Government-controlled city of Aleppo succeeded in reaching the suburb of Afrin, a small district due north, where thousands of children have taken refuge. The trucks were carrying a number of supplies including hygiene kits, blankets, water and high energy biscuits and would be distributed by volunteer teams from the Syrian Arab Red Crescent.

However, he added that UNICEF needed similar convoys to reach other children throughout the country and said that this would be increasingly difficult if the agency's budget shortfall were not addressed immediately. UNICEF has only received 50 per cent of the funds it has requested and still requires some $94 million by the end of the year.