RIYADH/ALEPPO — Leaders of Muslim countries are expected to suspend Syria's membership of the Organization of Islamic Cooperation at a summit in Mecca on Wednesday, despite the vocal objections of President Bashar al-Assad's main ally Iran.

The decision by the 57-member organization, which requires a two-thirds majority, will expose the divisions within the Islamic world over how to respond to civil war in a country that straddles the Middle East's main sectarian faultline.

In an apparent conciliatory gesture, Saudi state television showed King Abdullah welcoming leaders to the summit with Iran's President Mahmoud Ahmadinejad at his side. Abdullah and Ahmadinejad were shown talking and laughing together.

"It was a message to the Iranian nation and, I assume, to the Saudi people, that we are Muslim and we have to work together and forget about our differences," said Abdullah al-Shammari, a Saudi political analyst.

Syria's mainly Sunni Muslim rebels are backed by Sunni-ruled Arab states such as Saudi Arabia and Qatar as well as Turkey, while Shiite Iran supports Assad, a member of the Alawite minority sect, an offshoot of Shia Islam.

Those divisions have stymied diplomatic efforts to halt the bloodshed in Syria, where opposition sources say 18,000 people have been killed, and have raised the prospect of Syria becoming a proxy battlefield for outside powers.

Assad's former prime minister, Riyad Habib, a Sunni who defected this month, made his first public appearance on Tuesday since he fled, telling a news conference in Jordan that Assad controls less than a third of Syria and his power is crumbling.

"The regime is collapsing, spiritually and financially, as it escalates militarily," he said. "It no longer controls more than 30 percent of Syrian territory."

Hijab was not in Assad's inner circle, but as the most senior civilian official to defect, his defection after two months in the job was a blow to the president.

Hijab did not explain his estimate of the territory still controlled by Assad, whose military outnumbers and outguns the rebels fighting to overthrow him. The army is battling to regain control of Aleppo, Syria's biggest city, after retaking parts of Damascus that were seized by insurgents last month.

Curbs on media access make it hard to know how much of Syria is in rebel hands, but most towns and cities along the country's backbone, a highway running from Aleppo in the north to Deraa in the south, have been swept up in the violence, and Assad has lost swathes of land on Syria's northern and eastern border.

Hijab's defection prompted Washington to announce on Tuesday that it was removing him from a list of Syrian officials targeted by financial sanctions.

VIOLENCE

The fighting in recent weeks has been focused mainly on Aleppo, Syria's economic dynamo, where rebels have been holding out against government bombardment and air strikes.

Reuters journalists in Aleppo heard shelling and explosions in Saif al-Dawla district, next to the Salaheddine neighborhood which has seen some of the heaviest fighting in the last two weeks. One rebel fighter was killed by tank shelling, his bloodied body dragged out of the line of fire by comrades.

"We received some simple amounts of ammunition but it is not enough," said rebel fighter Hossam Abu Mohammad, a former army captain. "We need specific kinds of (anti-tank) weapons."

"We are about 600 Free Syrians fighting in Salaheddine and it is not enough," he told Reuters.

The violence has displaced 1.5 million people inside Syria and forced many to flee abroad, with 150,000 registered refugees in Turkey, Jordan, Lebanon and Iraq, U.N. figures show.

U.N. emergency relief coordinator Valerie Amos visited on Tuesday to discuss aid for civilians trapped or uprooted by the fighting, which has frequently prevented the delivery of food and medical supplies.

"She's there to express her grave, grave concern over the situation," spokesman Jens Laerke said. "She will look at the situation on the ground and discuss with the government and humanitarian partners how to scale up the response in Syria."