Militant Islamist fighters gesture as they take part in a military parade along the streets of Syria's northern Raqqa province. The fighters held the parade to celebrate their declaration of an Islamic "caliphate". Picture: Stringer

Damascus - The jihadist Islamic State (IS) took control of the key Syrian border town of Albu Kamal on Tuesday after a fierce three-day battle with rival fighters, a monitor said.

A spokesman for rebels fighting IS as well as President Bashar al-Assad's regime said the jihadists took over the town after pouring in reinforcements from neighbouring Iraq, where they have seized chunks of territory in a swift offensive.

The takeover comes two days after IS declared a “caliphate” in territory they seized in both Syria and Iraq, and ordered the world's Muslims to obey its leader Abu Bakr al-Baghdadi.

“The Islamic State took total control of Albu Kamal in (the oil-rich province of) Deir Ezzor, after fierce fighting pitting it against rebels backed by al-Qaeda affiliate Al-Nusra Front,” said the Syrian Observatory for Human Rights.

Omar Abu Leyla, the official rebel spokesman for Deir Ezzor province, told AFP “the battle was fierce... But IS has won this round.”

He said the jihadists won “after deploying major reinforcements from Iraq into Syria on Monday night”.

Fighting has raged for months in Deir Ezzor between the jihadists, formerly known as the Islamic State of Iraq and the Levant, against rebels backed by Al-Nusra Front.

The rebels held their ground for most of that time but the jihadists were bolstered “mainly because of the heavy weapons” captured from fleeing Iraqi troops during the lightening offensive, said Abu Leyla.

Hundreds of families fled Albu Kamal as IS took over, he said.

Meanwhile regime warplanes carried out four air strikes on Albu Kamal after IS seized the town, said the Observatory.

The Britain-based monitor also reported fighting in Shheil, an Al-Nusra Front bastion some 100 kilometres north of Albu Kamal.

Syria's war began as a peaceful revolt demanding Assad's ouster, but morphed into a conflict after the regime unleashed a massive crackdown on dissent.

Many months into the fighting, jihadists started streaming into Syria, and analysts have long warned of the conflict leading to a regional conflagration. - AFP