Warplanes unleashed a wave of air raids on rebel belts in Syria's north Tuesday even as the UN appealed for a nationwide ceasefire during the upcoming Muslim holiday of Eid al-Adha.

International peace envoy Lakhdar Brahimi issued the call for a truce as he travelled to Cairo as part of a regional tour to thrash out a possible solution to the conflict.

The morning air strikes around Maaret al-Numan were termed by the Syrian Observatory for Human Rights as the "most violent" since insurgents captured the strategic town last week.

The warplanes dropped bombs in a bid to break a rebel blockade of a highway, which is preventing army reinforcements from reaching second city Aleppo, theatre of intense fighting for the past three months, the group said.

"The bombing is targeting the villages of Hish, Maarshamsha, Maarshamrin, Talmans and Deir al-Gharbi in the vicinity of Maaret al-Numan," the Observatory said, adding rebels were responding with anti-aircraft fire.

Maaret al-Numan is strategically located in the northwest on the highway linking Damascus to Aleppo.

As it attempts to subdue the insurgency in the north, the army is also engaged in an attempt to put down rebels at Eastern Ghuta, in the countryside outside Damascus.

Brahimi, the UN-Arab League envoy for Syria, has called for a ceasefire during the upcoming Muslim holiday of Eid al-Adha, as the revolt is now in its 20th month with a death toll of more than 33,000.

Brahimi made his call as he shuttled between Syria's neighbours, which have been bitterly divided by the conflict along the confessional lines that have traditionally riven the Islamic world.

He was in Shiite-majority Iraq after talks in Shiite-ruled Iran, closest ally of the minority Alawite-dominated regime of Syrian President Bashar al-Assad.

Last week, Brahimi visited Saudi Arabia and Turkey, the two Sunni-led states which have been the greatest champions of the Syrian opposition. And on Tuesday he was in Cairo where he was to meet Arab League chief Nabil al-Arabi.

"Brahimi has appealed to the Iranian authorities to assist in achieving a ceasefire in Syria during the forthcoming Eid al-Adha, one of the holiest holidays celebrated by the Muslims around the world," a statement from the envoy said.

Eid al-Adha, which falls at the end of October, marks the climax of the annual hajj pilgrimage.

"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."

Iran proposed to Brahimi a political transition supervised by Assad, Deputy Foreign Minister Hossein Amir Abdolahian said, an idea unlikely to be acceptable to the opposition.

-- US call as Turkey-Syria tensions soar --

Tensions between Syria and Turkey have soared as Ankara and Damascus banned flights between the two countries after Ankara confiscated a cargo of radar equipment from a Syrian flight from Moscow last week.

On Monday, Turkish Prime Minister Recep Tayyip Erdogan reiterated that the cargo contained "war equipment."

"There is no point in diverting and saying it is radar equipment. Radar equipment functions as war equipment anyway," he said.

Ties between the two nations have deteriorated since rebels seized large swathes of territory along the long Syria-Turkey border.

On October 3, five Turkish civilians were killed by cross-border fire against the rebels that Syria charges are receiving arms from Gulf Arab states through Turkey.

The United States on Monday called on all Syria's neighbours to keep a careful watch over their airspace.

"Certainly we support the decision that Turkey has made in light of the apparent violation of their airspace by this aircraft," State Department spokeswoman Victoria Nuland said.

"We are encouraging all of Syria's neighbours to be vigilant with regard to how their airspace is used, particularly now that we have this concrete example."

Meanwhile the European Union imposed a new package of unilateral sanctions on Damascus on Monday, its 19th since the conflict erupted in March 2011.

But Western and European leaders are still facing an uphill task in getting Russia and China, key allies of Assad, on board. The two have repeatedly blocked action at the UN Security Council against the Assad regime.

British Foreign Secretary William Hague said on Monday: "I can't say that we made any progress."

Inside Syria fighting raged with at least 151 people killed on Monday, including 78 civilians.

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