Smoke rises from the Syrian town of Kobani, seen from near the Mursitpinar border crossing on the Turkish-Syrian border in the southeastern town of Suruc in Sanliurfa province October 14, 2014. REUTERS/Umit Bektas

By Humeyra Pamuk and Daren Butler

MURSITPINAR Turkey/ISTANBUL (Reuters) - American-led forces have sharply intensified air strikes in the past two days against Islamic State fighters threatening Kurds on Syria's Turkish border after the jihadists' advance began to destabilise Turkey.

The coalition had conducted 21 attacks on the militants near the Syrian Kurdish town of Kobani over Monday and Tuesday and appeared to have slowed Islamic State advances there, the U.S. military said, but cautioned the situation remained fluid.

U.S. President Barack Obama voiced deep concern on Tuesday about the situation in Kobani as well as in Iraq's Anbar province, which U.S. troops fought to secure during the Iraq war and is now at risk of being seized by Islamic State militants.

"Coalition air strikes will continue in both of these areas," Obama told military leaders from more than 21 coalition partners in the fight against Islamic State, including Turkey, Arab states and Western allies.

War on the militants in Syria is threatening to unravel a delicate peace in neighbouring Turkey where Kurds are furious with Ankara over its refusal to help protect their kin in Syria.

The plight of the Syrian Kurds in Kobani provoked riots among Turkey's 15 million Kurds last week in which at least 35 people were killed.

Turkish warplanes were reported to have attacked Kurdish rebel targets in southeast Turkey after the army said it had been attacked by the banned PKK Kurdish militant group, risking reigniting a three-decade conflict that killed 40,000 people before a cease-fire was declared two years ago.

Kurds inside Kobani said the U.S.-led strikes on Islamic State had helped, but that the militants, who have besieged the town for weeks, were still on the attack.

"Today there were air strikes throughout the day, which is a first. And sometimes we saw one plane carrying out two strikes, dropping two bombs at a time," said Abdulrahman Gok, a journalist with a local Kurdish paper who is inside the town.

"The strikes are still continuing," he said by telephone, as an explosion sounded in the background.

"In the afternoon, Islamic State intensified its shelling of the town," he said. "The fact that they're not conducting face-to-face, close distance fight but instead shelling the town from afar is evidence that they have been pushed back a bit."

Asya Abdullah, co-chair of the dominant Kurdish political party in Syria, PYD, said the latest air strikes had been "extremely helpful". "They are hitting Islamic State targets hard and because of those strikes we were able to push back a little. They are still shelling the city centre."

It was the largest number of air strikes on Kobani since the U.S.-led campaign in Syria began last month, the Pentagon said. The White House said the impact was constrained by the absence of forces on the ground but that evidence so far showed its strategy was succeeding.

CEASEFIRE THREATENED

The Turkish Kurds' anger and resulting unrest is a new source of turmoil in a region consumed by Iraqi and Syrian civil wars and an international campaign against Islamic State fighters.

The PKK accused Ankara of violating the cease-fire with the air strikes, on the eve of a deadline set by its jailed leader to salvage the peace process.

"For the first time in nearly two years, an air operation was carried out against our forces by the occupying Turkish Republic army," the PKK said. "These attacks against two guerrilla bases at Daglica violated the ceasefire," the PKK said, referring to an area near the border with Iraq.

Obama, who ordered the bombing campaign that started in August against Islamic State fighters, told the meeting of military leaders to expect a "long-term effort" in the battle against Islamic State militants.

"There will be days of progress and there are going to be some periods" of setbacks, he said.

Washington has faced the difficult task of building a coalition to intervene in Syria and Iraq, two countries with complex multi-sided civil wars in which most of the nations of the Middle East have enemies and clients on the ground.

In particular, U.S. officials have expressed frustration at Turkey's refusal to help them fight against Islamic State. Washington has said Turkey has agreed to let it strike from Turkish air base. Ankara has said that is still under discussion.

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