Islamic State jihadists pushed into the key Syrian town of Kobane on the Turkish border, 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 the IS group and its opponents, who include the United States and its Western and Arab allies.
Fresh air strikes by the US-led coalition on Tuesday hit positions held by Islamic State jihadists in the southwest of Kobane, according to an AFP journalist just across the border in Turkey.
Taking Kobane would give the IS organisation control of a long stretch of the Syria-Turkey border.
The jihadists launched their latest assault on Kobane after a three-week siege with a wave of suicide bomb attacks, Mustefa Ebdi, a Kurdish activist from the town, said 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.
Observatory director Rami Abdel Rahman said street battles were now being fought in the south and west of Kobane.
IS jihadists had "retreated by a few streets in the eastern areas they seized yesterday, but the fighting has now spread to the south and west of the town," Abdel Rahman said.
He said IS fighters had seized a number of buildings in the south and west of the town, including a hospital under construction on the western outskirts of Kobane.
Kobane activist Mustafa Ebdi said the latest air strikes had little effect.
"The strikes hit the Mishtenur area," he said, referring to a plateau south of Kobane.
"But they (IS) aren't gathered there. There are other places they should be hitting," he said.
Kurdish fighters meanwhile ordered all civilians to evacuate Kobane, Mustafa Bali, a spokesman for Kurds in the city, told AFP, adding that some 2,000 people had left the city.
In a sign of mounting desperation, a Kurdish female fighter blew herself up at an IS 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 jihadists, 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 northwestern Syria.
"She killed dozens of gang members and demonstrated the YPG fighters' determined resistance," her group said.
On another front, twin IS suicide truck bombings killed at least 30 YPG fighters and security officers on Monday in the Kurdish town of Hasakeh, northeast Syria, the Observatory said.
The jihadists 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, US 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.
'NATO to protect Turkey'
They also revealed 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," Kassig wrote.
IS began advancing on Kobane on September 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 IS, 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 IS attack.
"Turkey is a NATO ally and our main responsibility is to protect the integrity, the borders of Turkey," said NATO chief Jens Stoltenberg.
British media, meanwhile, reported that Turkish hostages freed by IS last month may have been released as part of a prisoner exchange for up to 180 jihadist 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 IS 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 IS 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-IS 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 jihadists 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 jihadists 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 percent of the total.