Fighters from the Islamic State group have seized all of Syria's main oil and gas fields, which are located in Deir Az Zor province next to Iraq, a monitoring group has said.

"IS took control of the Tanak oil field, located in the Sheiytat desert area in the east of Deir Ezzor province," late Thursday after rival rebels withdrew, said the Syrian Observatory for Human Rights on Friday.

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

Early on Thursday the group seized Syria's largest oil field - al-Omar oil field - on the Iraqi border, forcing the withdrawal of rival fighters.

They have still not captured the tiny al-Ward field, which produces about 200 barrels of oil per day and is in the hands of a local tribe, Observatory director Rami Abdel Rahman said.

The Nusra Front, which controlled al-Omar since late last year, abandoned the facility on Thursday without firing a bullet, the monitoring group said. 

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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.

In Deir Az Zor, the Islamic State - formerly known as the Islamic State of Iraq and Levant (ISIL) - has taken over nearly all the countryside. Its forces are bolstered by heavy weapons captured from Iraqi troops fleeing an offensive that it led.

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.

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

Nusra fighters also withdrew from Mayadeen and Shuhail in Deir Az Zor on Thursday, according to the Syrian Observatory. Local tribal fighters had pledged allegiance to the Islamic State, it added.