AMMAN (Reuters) - Islamic State fighters have seized the last major oilfield under Syrian government control during battles over a vast central desert zone, a group monitoring the conflict said on Monday.

The Jazal field was now shut down and clashes were ongoing east of Homs, with casualties reported on both sides, the Britain-based Syrian Observatory for Human Rights said, without giving dates or more details.

Syria's army said it had repulsed an attack in the same area but did not mention Jazal or comment on how much of the country's battered energy infrastructure remained under its sway. It said it killed 25 fighters, including non-Syrian jihadists.

"The regime has lost the last oilfield in Syria," said the Observatory, which tracks violence through a network of sources on the ground.

Commentators on social media said fighting had surged in the last two to three days and the rebels had taken the oilfield on Sunday.

Jazal is a medium-sized field that lies to the north west of the rebel-held ancient city of Palmyra, close to a region that holds Syria's main natural gas fields and multi-million-dollar extraction facilities.

The army, which has been fighting to retake the city and surrounding areas since they fell in May, had managed to secure the oil field's perimeter in June.

The Observatory also said U.S.-led coalition bombing raids in areas in the militant's de facto capital of Raqqa had killed at least 16 militants, including five foreign jihadists.

Islamist insurgents bombarded the heart of the Syrian capital Damascus and mortars killed at least one civilian and wounded scores, state television said.

(Reporting by Suleiman Al-Khalidi; Editing by Catherine Evans and Andrew Heavens)