Jihadists from the Islamic State group seized part of the key regime air base at Deir Ezzor in eastern Syria in fierce fighting on Saturday, a monitoring group said.

"IS mounted a dawn attack on Deir Ezzor military airport and entered several areas as fierce clashes unfolded between the jihadists and regime forces there," the Syrian Observatory for Human Rights said.

On Friday, the Britain-based group said the militants were closing in on the base, nearly surrounding its beleaguered garrison.

IS fighters control most of Deir Ezzor province, but half of its capital of the same name remains in government hands.

The oil-rich province lies between IS-controlled Raqa province and the border with Iraq, and is a key prize for the jihadist group which declared an Islamic "caliphate" straddling the two countries in June.