The Western-backed Syrian opposition group will begin a three-day meeting in Istanbul later Sunday to elect a new president and discuss the offensive by Islamic militants straddling Iraq and Syria, an official with the group said.

The meeting comes amid reports that 150,000 people have been displaced from their homes in eastern Syria by jihadi fighters who captured wide areas of the eastern province of Deir el-Zour in the past weeks.

Mustafa Osso said the Syrian National Coalition will pick a replacement for its current president, Ahmad al-Jarba, in a vote expected on Tuesday. He said the top two candidates for the job are senior coalition members Hadi Bahra and Muwaffaq Nairabiyeh, who belong to Jarba's Democratic bloc.

Jarba, who was elected in July of last year has already served two six-month terms -- the maximum period allowed by the coalition.

A statement by the group said that in addition to a vote for a new president, the coalition will also elect three vice presidents, a secretary general and a political committee. The statement said the coalition will be "discussing the military changes in Syria and the region in general, and its impact on the course of revolution."

Over the past weeks, Islamic militants launched a wide offensive in eastern Syria and northern Iraq capturing large areas on both sides of the border.

The group, which calls itself the Islamic State, six days ago declared the establishment of an Islamic state, or caliphate, in the territories it seized in Iraq and Syria. The group proclaimed its leader Abu Bakr al-Baghdadi the leader of its territory and demanded that all Muslims pledge allegiance to him.

Also Sunday, the Britain-based Syrian Observatory for Human Rights said the Islamic State group has displaced 150,000 Syrians from their homes in the eastern oil-rich province of Deir el-Zour. The group has been on the offensive in Deir el-Zour since late April and has captured a large numbers of towns and villages in the province after intense fighting with rival rebel groups.

On Saturday, the military chief of Syria's main Western-backed rebel group warned Syria risked a "humanitarian disaster" if allies do not send more aid to help his moderate forces halt the advance of the Islamic State group.

"We call on urgent support for the FSA with weapons and ammunition, and to avoid a humanitarian disaster that threatens our people," said Brig. Gen. Abdul-Ilah al-Bashir, commander of the Free Syrian army.