Islamic State militants are fighting street battles with Kurdish forces protecting the Syrian city of Kobani near the Turkish border.
Reports say that the jihadists broke into the east of the town using tanks and car bombs on October 6 and seized three districts in the city's east after fierce fighting with its Kurdish defenders.
The British-based Syrian Observatory for Human Rights said the IS militants wrested three areas in eastern Kobani -- the industrial zone, Maqtala al-Jadida, and Kani Arabane.
The observatory said the Kurdish defenders "had far fewer men and arms" than the attackers.
Kurdish fighters have ordered all civilians to evacuate Kobani.
Earlier in the day, the black flags of Islamic State jihadists were seen flying over buildings near the eastern edge of Kobani as the militant group advanced.
Kurdish defenders had called for the U.S.-led coalition against Islamic State to step up air raids to stave off the fall of the enclave, saying the strikes that took place over the last week were insufficient.
The extremist Sunni Muslim group has seized large swaths of Syria and Iraq, where it has been accused of carrying out widespread atrocities, including mass executions, abductions, torture, and forcing women into slavery.
Capturing Kobani would give IS a direct link between its positions in the Syrian province of Aleppo and its stronghold of Raqqa to the east.
It would also give the group full control of a large stretch of the Turkish-Syrian border.
The IS push to take Kobani, which started three weeks ago, has seen more than 185,000 civilians flee across the border, according to Turkish Prime Minister Ahmet Davutoglu.
Turkish security forces used tear gas October 6 to push dozens of reporters and Kurdish civilians away from the border zone, which has become increasingly dangerous because of stray mortar fire.
The parliament in Ankara last week authorized the government to join a U.S.-led campaign against IS, but so far no plans for military action have been announced.
Jens Stoltenberg, the new head of NATO, said on October 6 that the alliance would protect member state Turkey against any IS attack.
Meanwhile, the observatory said that on another front, twin IS suicide truck bombings killed at least 30 Kurdish Peshmerga fighters on October 6 in the Kurdish town of Hasakeh, in northeast Syria.
The U.S. Central Command said that in Syria, the U.S.-led coalition carried out anti-IS strikes on October 5-6 near Raqa, Deir Ezzor and Kobane, where two jihadist "fighting positions" were destroyed.
The Central Command said that in Iraq, they also launched three raids, targeting the jihadists near Fallujah and Ramadi, adding that Belgium and Britain took part in the strikes.
The Central Command also said that the U.S. military said had flown Apache helicopters against IS positions in Iraq for the first time, striking mortar teams and other units near the western town of Fallujah. The use of helicopters marks an escalation in the air war, putting American troops at higher risk.
It said the Pentagon has spent as much as $1.1 billion on U.S. military operations against IS in Iraq and Syria since the mission began in mid-June.
Separately, U.S. officials have said that Arab and other allied countries have carried out about 10 percent of the nearly 2,000 air raids against IS in Iraq and Syria since early August.