Kurdish forces battled overnight with Islamists trying to seize a hill overlooking a Syrian border town with Turkey as U.S.-led coalition warplanes carried out raids on the militants, a Kurdish official and a monitoring group said on Sunday.

A translator with the Kurdish Democratic Union Party (PYD) inside Kobani said Islamic State forces were hitting it with tank and mortar fire as they tried to seize Mistanour hill, a landmark whose capture would give them easy access to the town.

Kurdish forces had managed to stop Islamic State capturing the hill, Parwer Mohammed Ali told Reuters.

"Overnight there were new airstrikes. They struck three or four times in the vicinity Mistanour hill," he added.

Islamic State, a radical offshoot of al Qaeda, launched a new offensive to capture Kobani, a Kurdish town, two weeks ago as they try consolidate their hold on a stretch of territory across northern Syria and Iraq.

U.S-led air raids on Islamic State in Syria have done little to blunt its advance on Kobani, also known as Ayn al-Arab, and the violence has driven about some 180,000 Kurds into Turkey.

Turkey has shown no sign it will intervene to directly confront Islamic State on its borders. It sees the Kurdish armed groups defending Kobani as foes.

The Britain-based Syrian Observatory for Human Rights, which monitors the three-year-old Syrian war, said Islamic State has managed to capture the southern side of Mistanour hill, the furthest away from the town.

At least 11 Kurdish fighters and 16 Islamic State insurgents were killed in the overnight clashes, it said.

Ocalan Iso, deputy commander of the Kurdish forces defending Kobani, said the clashes had focused on the hill, which lies to the south east. Islamic State forces are now within a kilometre of Kobani, he said by telephone.

Spillover But No Reaction

Some violence has already spilled over the border. Early on Sunday a mortar round landed around 500 metres (550 yards) inside Turkish territory close to an army base at Mursitpinar.

Explosions were audible across the border from Kobani, where shells continued to land inside, a Reuters correspondent said.

On Saturday a Turkish special forces officer was wounded after being hit by shrapnel from a stray shell apparently fired by Islamic State fighters, according to media and local sources.

Two Turkish armoured vehicles were stationed at the Yumurtalik border crossing 5 km (three miles) west of Kobani on Sunday with their guns trained on Syria, but there was no sign of significant troop movements.

Tanks which earlier in the week had been deployed along the border had returned to their base.

Further west in Syria, government warplanes bombed towns in the countryside north of Aleppo, which the Syrian military is seeking to recapture from a mix of insurgent groups.

Last week the Syrian army made a new advance on Aleppo, seizing three villages north of the city and threatening rebel supply lines in a potentially major reversal.

President Bashar al-Assad's army has intensified an offensive in the heavily-populated western areas of Syria as U.S.-led warplanes concentrate on areas in the north and east - Islamic State areas which Damascus sees as less important.

Clashes took place between the Syrian army and Islamic State insurgents around Kowaires military airbase in Aleppo, the Observatory said. Syrian warplanes on Saturday carried out raids around the airport.

In the industrial city of Sheikh Najjar, northeast of Aleppo, Islamist groups including Nusra Front also fought with government forces backed by pro-government militias and fighters from Shi'ite Lebanese group Hezbollah, the Observatory said.