* IS-controlled buildings captured near council offices

* Militants have fought over Kobani for more than two months

* Thirteen militants killed - monitor (Adds details from Observatory)

BEIRUT, Nov 18 (Reuters) - Kurdish fighters captured six buildings used by Islamic State militants besieging the Syrian town of Kobani on Tuesday, and seized a large amount of the jihadist group's weapons and ammunition, a group monitoring the war said.

Islamic State has been trying to take control of the town, also known as Ayn al-Arab, for more than two months in an assault that has driven tens of thousands of Kurdish civilians over the border into Turkey and drawn strikes by U.S.-led forces.

The hardline Sunni Muslim movement, an offshoot of al Qaeda, has declared an Islamic caliphate covering large areas of land that it has captured in other parts of Syria and neighbouring Iraq.

Kurdish fighters seized six buildings used by Islamic State close to council offices in the north of the town and took a large quantity of rocket-propelled grenade launchers, guns and machine gun ammunition, the Britain-based Syrian Observatory for Human Rights said.

The buildings were in a strategic location close to Kobani's Security Square where the main municipal buildings are based, said Rami Abdulrahman who runs the Observatory, a group that tracks the conflict using sources on the ground.

The clashes killed around 13 Islamic State militants, including two senior fighters who had been helping to lead the militant group's assault on the town, he said.

Kurdish forces appear to have made other gains in recent days of fighting. Last week they blocked a road Islamic State was using to resupply their forces, the first major gain against the jihadists after weeks of violence.

"During the last few days we have made big progress in the east and southeast," said Idris Nassan, an official in Kobani.

Speaking by telephone, he estimated Islamic State controlled less than 20 percent of the town. Last month, officials said Islamic State controlled around 40 percent as it pushed further into the town.

The defence of Kobani has drawn in Kurdish peshmerga fighters from Iraq as well as Syrian rebel fighters. The U.S. military said on Monday it carried out nine strikes near Kobani since late last week, destroying seven Islamic State positions, four staging areas and one unit belonging to the group. (Reporting by Sylvia Westall; Editing by Andrew Heavens)