Islamic State al-Omar oil field Syria
Islamic State fighters are seen outside the al-Omar oil field in the eastern Deir al-Zour province, in Syria. YouTube

Militants, who declared an Islamic caliphate in the Middle East, now claimed to have seized Syria's largest oilfield.

Fighters of the Islamic State, previously known as Islamic State in Iraq and the Levant (Isis), said they took over the al-Omar oil field in the eastern Deir al-Zour province from rival rebel groups.

Video footage uploaded online showed armed Islamic State jihadist standing in front of the entrance of the field as the group's flag flew over a sign reading "Euphrates Oil Company - al-Omar field".

The footage could not be independently verified, but the Britain-based Syrian Observatory for Human Rights confirmed the field has fallen into the hands of the Islamic State.

The field was previously controlled by al-Qaeda affiliate in Syria, the al-Nusra Front that captured it from forces loyal to President Bashar al-Assad in November.

It was not immediately clear if the field is still fully operational and how much oil it produces. Prior to the civil war, Syria exported 370,000 barrels a day.

Al-Nusra and the Islamic state have been fighting each other over control of parts of Syria for months after the latter group was disowned by al-Qaeda in 2013.

The seizure of al-Omar came as the Islamic State reported a series of victories in the Deir al-Zour province and after a number of rebel factions in the area pledged their allegiance to the jihadists.

Activists in the region neighbouring Iraq said that towns and villages along the Euphrates River have been taken over by the Islamic State after al-Nusra troops withdrew.

The Deir el-Zour Coordination Committees, a collective of activists in the area, said that Islamic State militants had entered the towns of Mayadeen and Ishara.

The jihadist group led by Abu Bakr al-Baghdadi is now said to be in almost full control of a corridor stretching from the Syrian border town of Boukamal to the government-controlled provincial capital of Deir el-Zour to the northwest.

In videos posted on YouTube, the leaders of a number of rebel groups in the town of Sheheil are seen declaring they now back the Islamic State and its leader.

"We have decided to declare our allegiance to the Islamic State and the Caliph of the Muslims, Abu Bakr al-Baghdadi," a rebel leader says in one video, reading from a statement.

In another video, what appear to be fighters in the village of Ishara are seen announcing that they are ceasing their fight against the Islamic State.

"The clans of the city of Ishara, and the villages around it... and all of the factions in these areas... announce before God that they will cease fighting with the Islamic State," a man says as he stands in a courtyard with several other tribal elders.

The Syrian Observatory for Human Rights, said that fighting was still ongoing in the city but al- Nusra appeared about to collapse.

Earlier this week, after the group took over large parts of Iraq, Baghdadi announced the establishment of an Islamic caliphate, extending from the Syrian city of Aleppo to Diyala province in eastern Iraq and appointed himself caliph of the Muslim world.