DAMASCUS - Fighting raged in Syria's two biggest cities on Sunday as UN-Arab League envoy Lakhdar Brahimi ended his first visit to the country on a peace mission a rebel commander said was doomed to fail.

Iran admitted for the first time it has elite forces present in Syria and neighbouring Lebanon, where Pope Benedict XVI added his voice to calls for an end to the bloodletting, urging Arab states to propose workable solutions. As Brahimi departed after a four-day visit to the war-ravaged country, a rebel Free Syrian Army commander who had an Internet conference call with the envoy on Sunday said his mission would fail.

"We are sure Brahimi will fail like the other envoys before him, but we do not want to be the reason of his failure," the FSA chief for Aleppo province in north Syria, Colonel Abdel Jabbar al-Okaidi, told AFP by telephone.

"We discussed the general situation in Syria, mainly focusing on the destruction wielded by the regime on the country," said Okaidi, who talked to Brahimi along with the FSA spokesman in Syria, Colonel Qassem Saadeddine, and the group's chief in Damascus, Colonel Khaled Hobous. Brahimi, who replaced former UN chief Kofi Annan following the failure of his six-point peace plan, warned on Saturday after meeting President Bashar al-Assad that the worsening conflict threatens both the region and the world.

He insisted that "the solution can only come from the Syrian people." But Okaidi accused the international community of "giving political cover to the regime" and of pushing the opposition to hold talks with the regime but without pressuring the government to stop its repression.

In a rare news conference, the commander of Iran's Revolutionary Guards said members of his elite Quds Force were in Syria and Lebanon, but only to provide "counsel." "A number of Quds Force members are present in Syria and Lebanon... we provide (these countries) with counsel and advice, and transfer experience to them," Brigadier General Mohammad Ali Jafari said in Tehran.

"But it does not mean that we have a military presence there."

Western and Arab countries have accused Iran of giving military aid to Assad's regime.

As the fighting raged, Pope Benedict XVI celebrated mass in Lebanon, praying for peace and reconciliation.

"In a world where violence constantly leaves behind its grim trail of death and destruction, to serve justice and peace is urgently necessary," Benedict told an estimated 350,000 faithful at an open-air mass in Beirut.

"May God grant to your country, to Syria and to the Middle East the gift of peaceful hearts, the silencing of weapons and the cessation of all violence."

Despite the disruption to schooling in Aleppo and other flashpoint areas, Syrian state media announced "more than five million students and 385,000 teachers and employees" went back to school.

UNICEF described the return to school as an "immense challenge."

"For children, being back at school is one of the most effective ways of giving them a sense of stability, hope and normality," said spokeswoman Marixie Mercado.

"It really is a hugely important way of enabling children who have gone through a nightmare to see that they do have a future."

Hollywood actress Angelina Jolie, a UN refugee agency special envoy, visited a refugee camp in Iraq's autonomous Kurdistan region on Sunday and urged Syria's neighbours to keep their borders open to refugees.

The death toll from the conflict has risen to more than 27,000 people, according to the Observatory, which relies on activist accounts from the ground. The United Nations puts the toll at 20,000.