Fighters from the Islamic State group have seized control of Syria's largest oil field on the Iraqi border, forcing the withdrawal of rival fighters, Syrian activists say.

The Nusra Front, which has controlled al-Omar oil field since late last year, abandoned the facility on Thursday without firing a bullet, the UK-based Syrian Observatory for Human Rights said.

Al Jazeera cannot independently verify the Syrian Observatory's statement due to reporting restrictions inside Syria.

Live Box 2012108933861442

By seizing the al-Omar oil field, Islamic State, previously called the Islamic State in Iraq and the Levant (ISIL), now controls most oil and gas fields in the eastern province of Deir Az Zor and the surrounding countryside.

"Islamic State took control of the al-Omar oil field" located north of Mayadeen, the Syrian Observatory said, adding that the strategic town also was under the Islamic State's control since dawn on Thursday.

A video uploaded to YouTube shows a man identified as Commander Hammam saying: " We took it [the oil field] over without any fighting. They fled like rats."

Al Jazeera cannot independently verify the authenticity of the video.

Al Jazeera's Rula Amin, reporting from Beirut in neighbouring Lebanon, said the Islamic State now controlled "a huge expanse of land about five times the size of Lebanon" and oil assets that could produce 35,000 barrels a day.

Al-Omar field has changed hands several times in the course of Syria's three-year conflict.

Nusra Front and several other allied factions captured it from forces loyal to President Bashar al-Assad in November.

Nusra Front fighters also withdrew from two towns in Deir Az Zor on Thursday, according to the Syrian Observatory.

Nusra pulled out of Mayadeen and Shuhail, its regional stronghold, while local tribal fighters had pledged allegiance to the Islamic State.

Most of the border province is now under the control of advancing forces of the Islamic State.

Earlier this week the Islamic State seized the town of Albu Kamal on the Iraqi frontier from Nusra, securing both sides of the border crossing.

Source: Al Jazeera and agencies