SURUC, Turkey (AFP) - Islamic State of Iraq and Syria (ISIS) militants pushed into the key Syrian town of Kobane on the Turkish border on Monday, seizing three districts in the city's east after fierce street fighting with its Kurdish defenders.

Kobane, also known as Ain al-Arab, has become a strategic battleground between ISIS and its opponents, who include the United States and its Western and Arab allies. Taking Kobane would give ISIS control of a long stretch of the Syria-Turkey border.

The militants launched their latest assault on Kobane after a three-week siege with a wave of suicide bomb attacks, said Mr Mustefa Ebdi, a Kurdish activist from the town, on his Facebook page.

After penetrating the city, they waged street battles against Kurdish defenders, sending hundreds of civilians fleeing towards the Turkish border, the Syrian Observatory for Human Rights said. "They have taken the industrial zone, Maqtala al-Jadida and Kani Arabane in eastern Kobane after violent combat with Kurdish People's Protection Units (YPG) fighters" who had far fewer men and arms, said the Observatory.

Kurdish fighters meanwhile ordered all civilians to evacuate Kobane, said Mr Mustafa Bali, a spokesman for Kurds in the city, adding that some 2,000 people had left the city.

The ISIS advances came after an AFP photographer saw two black flags of the group flying on Kobane's eastern side.

In a sign of mounting desperation, a Kurdish female fighter blew herself up at an ISIS position east of Kobane on Sunday, the Observatory said.

It was the first reported instance of a female Kurdish fighter employing a tactic often used by the militants, said the Britain-based monitor, which relies on a network of sources inside war-ravaged Syria for its reports.

The bomber, in her 20s, was a full-time YPG fighter identified as Dilar Gencxemis, alias Arin Mirkan, from Kurdish-controlled Afrin in north-western Syria.

"She killed dozens of gang members and demonstrated the YPG fighters' determined resistance," her group said.

On another front, twin ISIS suicide truck bombings killed at least 30 YPG fighters and security officers on Monday in the Kurdish town of Hasakeh, north-east Syria, the Observatory said.

The militants had sparked further outrage at the weekend with the release of a video showing the beheading of Briton Alan Henning.

The video - the latest in a series of on-camera beheadings of Western hostages - also included a threat to another hostage, United States aid worker Peter Kassig.

His parents have issued a video plea for their son's release, urging his captors to show mercy towards the 26-year-old former US soldier who has converted to Islam.

They also revealed Mr Kassig had sent them a letter in June.

"I am obviously pretty scared to die but the hardest part is not knowing, wondering, hoping and wondering if I should even hope at all," Mr Kassig wrote.

ISIS began advancing on Kobane on Sept 16, seeking to cement its grip over a long stretch of the border. The offensive prompted a mass exodus, with some 186,000 people fleeing into Turkey.

The Turkish security forces used tear gas Monday to push dozens of reporters and Kurdish civilians away from the border zone, which has become increasingly dangerous because of stray mortar fire.

Parliament in Ankara last week authorised the government to join a US-led campaign against ISIS, but so far no plans for military action have been announced.

The new head of Nato on Monday vowed to protect member Turkey against any ISIS attack.

"Turkey is a Nato ally and our main responsibility is to protect the integrity, the borders of Turkey," said Nato chief Jens Stoltenberg.

The British media, meanwhile, reported that Turkish hostages freed by ISIS last month may have been released as part of a prisoner exchange for up to 180 fighters.

The Times newspaper cited a list it had received saying that among them were three French nationals, two British, two Swedes, two Macedonians, one Swiss and one Belgian.

Extremist Sunni Muslim group ISIS has seized large parts of Syria and Iraq, where it has been accused of carrying out widespread atrocities, including mass executions, abductions, torture and forcing women into slavery.

After first launching strikes against ISIS in Iraq in August, Washington has built a coalition of allies to wage an air campaign against the group.

In Syria, the coalition carried out anti-ISIS strikes on Sunday and Monday near Raqa, Deir Ezzor and Kobane, where two jihadist "fighting positions" were destroyed, said US Central Command.

In Iraq, they also launched three raids, targeting the militants near Fallujah and Ramadi, it said, adding Belgium and Britain took part in the strikes.

On Monday, officials said the US military has started flying attack helicopters against the militants in Iraq for the first time, marking an escalation in the air war that puts American troops at higher risk.

In all, nearly 2,000 air raids had been launched by the coalition in both Iraq and Syria, US defence officials said.

Some 1,768 air strikes were carried out by US warplanes while other coalition aircraft were responsible for 195 others, or about 10 per cent of the total.