Kurdish militiamen backed by US-led air strikes were locked in fierce fighting to prevent the besieged border town of Kobane from falling to IS group fighters.
"There are real fears that the IS may be able to advance into the town of Kobane itself very soon," the Syrian Observatory for Human Rights warned.
The Britain-based watchdog reported fresh US-led air strikes on the advancing jihadists overnight after the heavily outgunned Kurdish fighters were forced to fall back west and southeast of the town, also known as Ain al-Arab.
The US-led coalition had already carried out at least seven strikes on IS targets around Kobane over the five days to Wednesday, US Central Command said.
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Observatory director Rami Abdel Rahman said there were concerns over "the Kurds' capacity to resist, as the IS are using tanks and other heavy weaponry in their attack."
IS seized large stocks of heavy weaponry from fleeing troops when they captured Iraq's second city of Mosul in June. They took more when they overran the Syrian garrison at Tabqa air base south of Kobane in late August.
NATO member Turkey, after months of caution, has decided to harden its policy, with parliament due to vote later today on a government request to authorise military action against IS in both Iraq and Syria.
Ankara has not yet indicated what form its assistance could take although President Recep Tayyip Erdogan has repeatedly called for a buffer zone on the Turkish border inside Syria to ensure security.
According to the Hurriyet newspaper, the request seeks permission from parliament for the presence and transit of foreign soldiers in Turkish territory as well as the deployment of Turkish military forces to Iraq or Syria.