Turkey's president said on Tuesday the Syrian Kurdish town of Kobani was "about to fall" as Islamic State fighters pressed home a three-week assault that had cost a reported 400 lives and forced thousands to flee their homes.

The prospect that the town could be captured by Islamic State, who are now within city limits, has increased pressure on Turkey to join an international coalition to fight against the jihadists.

Islamic State wants to take Kobani in order to strengthen its grip on the border area and consolidate the territorial gains it has made in Iraq and Syria in recent months. U.S.-led air strikes have so far failed to prevent its advance on Kobani.

Turkey said it was pressing Washington for more air strikes, although President Tayyip Erdogan said bombing was not enough to defeat Islamic State, and he set out Turkey's demands for additional measures before it could intervene.

"The problem of ISIS (Islamic State) cannot be solved via air bombardment. Right now Kobani is about to fall," he said during a visit to a camp for Syrian refugees.

"We had warned the West. We wanted three things. No-fly zone, a secure zone parallel to that, and the training of moderate Syrian rebels," he said.

He said Turkey would take action if there were threats to Turkish soldiers guarding a historic site in Syria that Ankara regards as its territory. But so far Turkey has made no move to get involved in the fighting across the border.

U.S. Secretary of State John Kerry and Turkish Prime Minister Ahmet Davutoglu had spoken twice in recent days to discuss the situation, State Department spokeswoman Jen Psaki said.

"Turkey is determining what larger role they will play," Psaki told a daily briefing. "They have indicated their openness to doing that, so there is an active conversation about that."

Retired U.S. General John Allen, the envoy charged with building the coalition against Islamic State, and his deputy Brett McGurk will visit Turkey later this week for talks.

U.N. Syria envoy Staffan de Mistura said Turkey had been generous in receiving refugees from Kobani but the international community needed to protect the town. "What is needed now is concrete action," he said, without elaborating.

France said it was vital to stop Islamic State's advance on Kobani, and was discussing with Turkey what could be done. "A tragedy is unfolding, and we must all react," Foreign Minister Laurent Fabius told parliament. But some analysts doubt the will exists among Western allies to take further action.

"It's the coalition of the unwilling, each country is doing the bare minimum, particularly in Syria," said Fadi Hakura at the London-based think tank Chatham House.

From across the Turkish border, two Islamic State flags could be seen flying over the eastern side of Kobani.

The Syrian Observatory for Human Rights monitoring group said it had documented 412 deaths of civilians and fighters during the three-week battle for Kobani.

The U.S. military said it and allied air forces launched strikes on Islamic State in Syria on Monday and Tuesday. In the Kobani area the raids destroyed armed vehicles, a tank and a vehicle carrying anti-aircraft artillery.

On the ground, a burning tank, apparently belonging to Islamic State, could be seen on the western edge of town. There were also clashes on the northern fringe and mortar explosions could be heard to the northeast.

Islamic State fighters were using heavy weapons and shells to hit Kobani, senior Kurdish official Asya Abdullah told Reuters from inside the town, estimated by the U.N. on Tuesday to contain possibly a few hundred remaining residents.