Dramatically increased US-led airstrikes have killed hundreds of "Islamic State" (IS) jihadists around the besieged Syrian border city of Kobani, US defense officials announced.

The US-led coalition has carried out nearly 40 airstrikes hitting targets in and around Kobani in the last two days, marking the largest number of targets hit since the bombings began on September 22. Kurdish fighters reportedly had given the Americans coordinates of IS targets to strike, though US defense officials would not confirm this.

Pentagon spokesman John Kirby said the situation around Kobani remains fluid, but that Kurdish fighters still control the town.

"The more [IS] wants it, the more resources they apply to it, the more targets we have to hit," Kirby said. "We know we've killed several hundred of them."

Kirby added that the airstrikes have degraded IS' ability to conduct operations and said IS would have difficulty reinforcing their ranks. "It's not like they have a whole heck of a lot of ability to reconstitute that," he said.

United States Central Command said the recent round of airstrikes destroyed several IS positions and hit 16 buildings occupied by IS militants.

The airstrikes have reportedly lifted the spirits of the Kurdish fighters around Kobani, and the Syrian Observatory for Human Rights said the strikes have been increasingly coordinated. "(The airstrikes) are more serious than before because the coordination has grown in the last six days," said Rami Abdulrahman, who runs the observatory.

US-Turkey relations strained

Meanwhile, Turkey launched airstrikes against Kurdish rebels inside its borders, defying calls from Washington to instead focus on the IS.

The strikes were the first against Kurdistan Workers' Party (PKK) targets since a March 2013 ceasefire.

There had been increasing tension between Turkey and the PKK over Ankara's refusal to allow Turkish Kurds to leave the country to defend Kobani.

The fresh conflict between Ankara and the PKK also highlights the complicated position Turkey faces as it negotiates its role with the US and NATO allies fighting the IS.

The strikes came ahead of an October 15 deadline set by PKK leader, Abdullah Ocalan, for a roadmap to salvage the fragile peace process.

bw/lw/crh (Reuters, AFP, AP)