* Strategic border town besieged by Islamic State
* Kurdish forces call for U.S.-led air strikes
* More than 140,000 Syrian Kurds have fled to Turkey (Adds U.N. begins airlifting aid into Turkey)
By Rasha Elass and Jonny Hogg
BEIRUT/KARACA, Turkey, Sept 25 (Reuters) - Kurdish forces in northern Syria pushed back an advance by Islamic State fighters towards a strategic town on the Turkish border on Thursday and appealed for U.S.-led air strikes to target the insurgents' tanks and heavy armaments.
Islamic State launched a new offensive to try to capture the border town of Kobani more than a week ago, besieging it from three sides. At least 140,000 Kurds have fled the town and surrounding villages since Friday, crossing into Turkey.
The U.N. refugee agency said the first of eight planned aid flights landed in the southern Turkish city of Adana, carrying pre-fabricated warehouses, sleeping mats, plastic sheeting and cooking utensils to help Turkey cope with the influx.
Kurdish and Islamic State fighters exchanged artillery and machinegun fire in a cluster of villages about 15 km (9 miles) west of Kobani, where the frontline appeared not to have moved significantly for several days, a Reuters witness said.
Kurdish officials meanwhile said Islamic State had concentrated their fighters south of the town late on Wednesday and had pushed towards it, but that the main Kurdish armed group in northern Syria, the YPG, had repelled them overnight.
"The YPG responded and pushed them back to about 10-15 km (6-9 miles) away," Idris Nassan, deputy minister for foreign affairs in the Kobani canton, told Reuters by telephone.
Syrian Kurdish refugees watching the fighting from a hill on the Turkish side of the border said the Islamic State insurgents had not been able to advance from positions they had taken up in olive groves west of Kobani.
Turkish military vehicles patrolled their side of the border, with soldiers occasionally moving people away from the hill overlooking the fighting. Heavy weapons fire could also be heard further away from the border inside Syrian territory.
The town's location has been blocking the Sunni Muslim insurgents from consolidating their gains in northern Syria. The group tried to take the town in July but was repulsed by local forces backed by Kurdish fighters from Turkey.
The YPG on Thursday renewed calls for U.S.-led air strikes to hit Islamic State positions around Kobani.
"Although all ISIS positions and their heavy armaments, including tanks and armored vehicles around Kobani, are clear and within view for everyone on the front line, it is worth noting that these targets have not been bombed yet," YPG spokesman Redur Xelil said.
"We are of the utmost readiness to cooperate with the international coalition forces against terrorism and give it detailed information about the main targets," he said.
A thin but steady stream of people continued to cross from Syria into Turkey at the Yumurtalik border crossing after it reopened on Thursday, with police carrying out security checks. An estimated 1,300 people crossed into Turkey on Wednesday alone, and the total number since the fighting began near Kobani had risen to 144,000, according to the United Nations.
Turkey is already sheltering more than 1.3 million refugees from Syria. The U.N. refugee agency was helping the Turkish authorities set up two new camps to cope with the influx, UNHCR spokeswoman Selin Unal said.
Turkey has been slow to join calls for a coalition to fight Islamic State in Syria, worried in part about links between Syrian Kurds and the Kurdistan Workers Party (PKK), a militant group which waged a three-decade campaign against the Turkish state for greater Kurdish rights.
The PKK has called on Turkey's Kurds to join the fight to defend Kobani and accused Ankara of supporting Islamic State. Residents in the border area say hundreds of youths have done so, although Turkish security forces have been trying to keep them from crossing the frontier.
Turkey strongly denies it has given any form of support to the Islamist militants but Western countries say its open borders during Syria's three-year civil war allowed Islamic State and other radical groups to grow in power.
Ocalan Iso, a Kurdish defense official, confirmed that YPG forces had stemmed Islamic State's advances south of Kobani, known as Ayn al-Arab in Arabic.
"As our fighters secured the area, we found 12 Islamic State bodies," he said by telephone. Islamic State fighters also remain to the east and west of the town and fighting continues in the south.
Both men said they had also heard warplanes flying over Kobani late on Wednesday for the first time, but it was not clear exactly which areas they were targeting.
A third night of U.S.-led air strikes on Wednesday targeted Islamic State-controlled oil refineries in eastern Syria, U.S. officials said.
The British-based Syrian Observatory for Human Rights, which monitors the war, said eight YPG fighters had been killed in overnight clashes. (Additional reporting by Tom Miles in Geneva; Editing by Nick Tattersall and Angus MacSwan)