IS jihadists, who proclaimed a "caliphate" in June straddling territory captured in Iraq and Syria, launched their offensive for the town -- also known as Ain al-Arab -- in mid-September.
"At least 1,013 people have been killed in fighting in Ain al-Arab from the beginning of the offensive till last night," said Syrian Observatory for Human Rights director Rami Abdel Rahman.
Militants from the Sunni extremist IS group accounted for 609 of those killed in the Kurdish town on the Turkish border, he said.
There were 24 civilians among the dead, said the director of the Britain-based group which relies on a network of sources on the ground for its information.
The toll for jihadists excludes those killed in US-led strikes on the Islamic State group.
Syrian Kurdish forces have been battling to repel IS militants from Kobane since September 16.
The fighters from the town have been joined by Syrian rebels who have fought both President Bashar al-Assad's regime and IS, as well as by Iraqi Kurd peshmerga forces.
