Islamic State (Isis) militants have made fresh gains, taking at least 40% of the Syrian Kurdish town of Kobani and capturing its Kurdish headquarters.
Jihadists have almost overrun the Kurdish security quarters which house the administrative buildings used by the local government, according to the British-based Syrian Observatory for Human Rights.
If confirmed, the attack would represent the most significant Isis advance since it launched an assault on Kobani and sent 150,000 Syrian Kurds fleeing to Turkey.
The attack also came after the US-led coalition stepped up its aerial bombardments of Isis positions near Kobani, in an effort to break the siege and allow Kurdish forces stranded there to recover.
The military effort is combined with a new push to convince Turkey to join in military operations. Retired marine general John Allen and Nato chief Jens Stoltenberg met Turkish officials in a bid to heap pressure on the Nato member states to intervene.
The standoff over Kobani also threatens to endanger the fragile peace process between Turkey and the PKK.
Turkey has been reluctant to join the US-led coalition against IS despite its allies' insistence. Its main concern was linked to the possibility of Turkish and Syrian Kurds fighting together against IS, laying out the foundations for a future independent state.
Turkish leaders have proposed a border buffer zone to include ground troops and a no-fly zone over the area, but those suggestions have been met coldly by Kurds and Americans.
Turkish officials have said they do not want to take on a greater role until the coalition lays out a strategy to oust Syrian president Bashar al-Assad. Turkey was one of the first countries to condemn the regime in the aftermath of 2011 pro-democracy demonstrations, supporting rebel groups in Syria.
However, US state department spokeswoman Jen Psaki said attacking Assad "is not the focus of our international coalition and not the focus of efforts by the United States".
"The capture of the headquarters will allow the jihadists to advance on the border post with Turkey to the north of the town," the Observatory told AFP.
"If they achieve that, they will have the Kurdish forces inside Kobani completely surrounded."