MURSITPINAR, Turkey/BEIRUT (REUTERS) - The United States and its allies have dramatically stepped up air strikes in the past two days near the Syrian town of Kobane, where Kurdish defenders said they had given the Americans target coordinates to try to halt an ISIS (Islamic State in Iraq and Syria) assault.

The U.S.-led military coalition said it had bombed ISIS targets in and around Kobane nearly 40 times in the space of 48 hours, around triple the pace of last week.

A four-week siege of the mainly Kurdish town on the border with Turkey has become a focus of the U.S.-led effort to halt the militants, who have seized swathes of territory in Syria and Iraq. The United Nations has warned of a massacre if the town falls to the militants, who now control nearly half of it.

The coalition has been bombing ISIS targets in Iraq since August and extended the campaign to Syria in September. After weeks in which Kobane was rarely targeted, the town has become the main focus of strikes.

The Pentagon declined to confirm any coordination with the main Kurdish armed group, YPG. "I just don't have any details to announce or speak to with respect to coordination on the ground," U.S. Rear Admiral John Kirby, the Pentagon's press secretary, told reporters when asked about the report.

Kirby said the situation in Kobane was fluid but that the Pentagon believed Kurdish fighters still held the town.

In the two 24-hour periods since Monday, the US military reported 21 and 18 strikes on militant targets in or near the town, which is called Ayn al-Arab in Arabic. Last week it typically struck the area just six or seven times per day.

A monitoring group said the strikes had also become more effective, killing at least 32 ISIS fighters in direct hits this week.

Kurdish officials said the YPG had begun giving the coordinates of Islamic State positions to the U.S.-led alliance. "The senior people in YPG tell the coalition the location of ISIL targets and they hit accordingly," YPG spokesman Polat Can told Reuters, using another acronym for ISIS. "Some of (the militants) have withdrawn, but they regroup and return. But because the air strikes are working in coordination, they hit their targets well," he said.

He did not disclose how the YPG fighters were sharing the coordinates.

Tim Ripley, a British expert with Jane's Defence Weekly, said US air controllers responsible for picking targets could check any information provided by YPG fighters by also using spotters watching the fighting from across the frontier in Turkey, as well as video relayed by drones.

The YPG forces have been struggling to defend Kobane from the better armed ISIS fighters who have used tanks, artillery and suicide truck bombs.

Kobane appeared close to falling a week ago as ISIS entered its eastern and southern districts and raised its black flag. As recently as Saturday, Kurdish leaders were calling for the air strikes to be stepped up.

In recent days, as the air strikes have increased, the militants have made little progress. The Kurds say they have taken back areas on the west of the town.

US President Barack Obama expressed deep concern on Tuesday about the situation in Kobane as well as in Iraq's Anbar province which US troops fought to secure during the Iraq war.

The intensified air campaign around Kobane has lifted the spirits of Kurds who have maintained a vigil watching the fighting from a hilltop just over the border in Turkey.

Dozens cheered as a powerful air strike hit eastern Kobane on Wednesday afternoon, sending up a plume of smoke.

The Syrian Observatory for Human Rights, which tracks the war using a network of sources on the ground, said one of the allied air strikes in the last day had killed a group of ISIS fighters just 50 metres from a Kurdish position. Another had destroyed a two-storey building used by the militants.

Rami Abdulrahman, who runs the Observatory, said seven ISIS fighters had been killed in clashes with the Kurds on Wednesday, along with at least six on the Kurdish side. "(The air strikes) are more serious than before because the coordination has grown in the last six days," Abdulrahman said.

The town's plight has angered Kurds across the border in Turkey, who accuse the Ankara government of doing too little to help protect their kin in a battle that has unfolded within view of Turkish tanks at the frontier. At least 35 people died in clashes last week between Turkish Kurds and the police.

Turkey has taken in 200,000 refugees from the area but has rejected the Syrian Kurds' request to open a land corridor so they can resupply the besieged town with arms and fighters from other parts of northern Syria.

Abdulrahman Gok, a journalist inside Kobane, said the latest air strikes had allowed the YPG to make some gains.

"Following the air strikes, I went to the last safe point in eastern side of the city. Some buildings that had been occupied by ISIS fighters were empty," he said. "On the west, YPG destroyed a vehicle that belonged to ISIS and killed the militants inside."