BEIRUT (Reuters) - Kurdish fighters captured six buildings from Islamic State militants besieging the Syrian town of Kobani on Tuesday and seized a large haul of their 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 neighboring Iraq.

The six buildings seized by Kurdish fighters from Islamic State were in a strategic location in the town's north, close to Security Square where the main municipal offices are based, said Rami Abdulrahman, who runs the Observatory, a group that tracks the conflict using sources on the ground.

The Kurds also took a large quantity of rocket-propelled grenade launchers, guns and machine gun ammunition.

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 its 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.

But Islamic State still appeared to be holding a significant grip on the town. Abdulrahman estimated it controlled more than 50 percent of the city.

The Observatory said Islamic State also fired at least 13 shells at Kurdish positions in Kobani, even as U.S.-led coalition planes flew overhead.

The defense 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 Angus MacSwan)