"The (Syrian Kurdish) People's Protection Units (YPG) targeted IS positions behind the (strategic) Mashta al-Nur hill and cut off the Halanj-Ain al-Arab road south of the town," said the Syrian Observatory for Human Rights, using the Arabic name Ain al-Arab for Kobane.
It said 16 IS members were killed in the advance.
The YPG has been fighting the IS since mid-September in Kobane. The Syrian Kurds are backed by Iraqi Kurdish peshmerga fighters and Syrian rebels.
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Kobane has become a major symbol of resistance against the IS, which in recent months has seized large swathes of territory in Iraq and Syria.
A US-led coalition on September 23 launched air strikes against IS positions in Syria, including around Kobane.
On Wednesday, the Observatory said at least 865 people, 746 of them IS fighters, have been killed in the strikes.
Another 50 were civilians, including eight children, while 68 of those killed were members of Syria's Al-Qaeda affiliate.