Syria has said it is prepared to explore a truce proposal by international peace envoy Lakhdar Brahimi, even as it unleashed multiple air strikes on rebel positions on a key highway.

The exiled opposition said Tuesday it would welcome any ceasefire but that the ball was in the government's court to halt its daily bombardments.

The Syrian Observatory for Human Rights said that pre-dawn air raids around Maaret al-Numan were the "most violent" since insurgents captured the strategic town on the Damascus-Aleppo highway last week.

The Syrian foreign ministry said early Tuesday that it looked forward to talks with UN-Arab League envoy Brahimi on his proposal for a ceasefire for the four-day Muslim Eid al-Adha holiday beginning at the end of October, which he has been promoting on a regional tour.

But spokesman Jihad Maqdisi stressed that the rebels and their backers would also need to be involved.

"In order to succeed in any initiative, it takes two sides," Maqdisi said in answer to a question from AFP.

"The Syrian side is interested in exploring this option and we are looking forward to talking to Mr Brahimi to see what is the position of other influential countries that he talked to in his tour," he said.

"Will they pressure the armed groups that they host and finance and arm in order to abide by such a ceasefire?"

The opposition Syrian National Council said it would expect the rebel Free Syrian Army to reciprocate any halt to the violence but that it expected the government to act first.

"We would welcome any halt to the killings but we think the appeal needs to be addressed first to the Syrian regime, which has not stopped bombarding Syrian towns and villages," SNC leader Abdel Basset Sayda told AFP.

Rebel fighters "are only acting in self-defence, so it is normal that they would halt hostilities when the war machine does so", he added.

Brahimi was in Cairo on Tuesday on the latest leg of a swing that has already taken him to Turkey and Saudi Arabia, staunch backers of the opposition, and to Iran, Syria's closest ally.

Brahimi's office said the envoy had appealed for Iranian help to broker the truce.

"He reiterated the call by UN Secretary General Ban Ki-moon for a ceasefire and a halt to the flow of arms to both sides. A ceasefire, he said, would help create an environment that would allow a political process to develop."

The UN chief had previously called for a unilateral government ceasefire to be matched by the rebels afterwards, but that idea was rejected by Damascus as its troop losses mount.

-- 'Cannot be mere spectators' --

Warplanes targeted the rebel blockade of the highway to Aleppo, theatre of intense fighting for the past three months, the Observatory said, adding that rebels responded with anti-aircraft fire.

"Since this morning, there have been 29 air strikes on the area of Maaret al-Numan," said Observatory chief Rami Abdel Rahman. "Most civilians from the area have fled."

Army shelling of nearby Kafr Nabal killed two children, aged six and 10, said the Observatory, adding that they were among at least 78 people who died in bloodshed around the country.

Another five children under the age of six, and two adults, died in shelling of homes at Mayadeen village in the eastern province of Deir Ezzor, added the Britain-based group.

The Observatory -- which relies on a network of activists, medics and lawyers for its information -- says children account for 2,300 of the 33,000 people killed in the conflict.

Pope Benedict XVI will send a delegation to the Syrian capital to "express his brotherly solidarity with the entire population", the Vatican said.

"We cannot be mere spectators to the tragedy taking place in Syria," the Holy See's number two official, Cardinal Tarcisio Bertone, said during a global synod of Roman Catholic bishops at the Vatican.

Syria has a significant Christian minority, some of them Catholic.

A UN commission investigating rights abuses in the wartorn country warned that foreign militants fighting in Syria "could contribute to an increased "radicalisation".

"The presence of foreign militants, radical Islamists or jihadists, worries us very much," commission head Paulo Sergio Pinheiro told reporters in New York, estimating there were hundreds of foreign combatants on the ground in Syria.

"Their presence can contribute to radicalisation... this presence is particularly dangerous in a very volatile conflict," he said.

Pinheiro added that the commission feared the foreign combatants were not fighting for "the building of a democratic state in Syria" but "for their own agenda".

In other developments, the UN food agency said prices for basic provisions had nearly doubled in Syria since the conflict erupted in March last year, and that it had failed to deliver supplies to 100,000 people because of the spiralling fighting.

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