Syria's former Pime Minister, who defected last week, said on Tuesday that the regime was collapsing and now only controlled about a third of the conflict-wracked country.

"The Syrian regime only controls 30 per cent of Syria's territory. It has collapsed militarily, economically and morally," Riad Hijab told a news conference in the Jordanian capital Amman.

Hijab said he had on August 5 decided to quit his post and head for Jordan, the most high profile in a string of defections from President Bashar al-Assad's regime, which is becoming increasingly embattled as the almost 17-month conflict shows no signs of abating.

Rebels claimed on Monday they had shot down a Syrian fighter jet and captured its pilot in what would be a major coup as it battles escalating air attacks by government forces, particularly in the key northern city of Aleppo.

The government was also dealt a major blow last month when three top security officials at the heart of Assad's Alawite-led inner circle were killed in a bomb attack in Damascus claimed by the rebel Free Syrian Army.

"Syria is full of officials and military leaders who are awaiting the right moment to join the revolt," Hijab said, while urging Syria's fractured opposition to unite.

A number of senior political and military figures have abandoned the regime but until military units begin to defect en masse, analysts say the ruling core - the Assad family and the top echelon of the military and security services - remains intact.

Hijab's comments came as fresh fighting for control of key districts of Aleppo erupted while Syrian forces bombarded areas around Damascus and launched a new security operation in the capital.

With Assad increasingly under pressure, a top Presidential aide was dispatched on Tuesday to China, which has said it wants an immediate ceasefire and political dialogue to halt the bloodshed.

China and Russia are at odds with the West over how to end the fighting, after both traditional Syria allies vetoed UN Security Council resolutions.

China said it had backed the peace plan of former peace envoy Kofi Annan, who quit earlier this month in the face of the continued violence and deadlock among world powers.

Assad advisor Bouthaina Shaaban is to hold talks with Foreign Minister Yang Jiechi, while Beijing has said it was also considering inviting members of the Syrian opposition to visit.

UN humanitarian chief Valerie Amos also arrived in Damascus on a regional visit "to draw attention to the deteriorating humanitarian situation" and discuss ways of scaling-up relief efforts.

The conflict has killed over 23,000 people since March last year, according to a rights watchdog, while the UN says over one million people have been displaced and another 140,000 have fled to Syria's neighbours.

Damascus also faces isolation by fellow Muslim states as the Organisation of Islamic Cooperation (OIC) meets in the Saudi City of Mecca to discuss a recommendation for Syria to be suspended from the 57-nation body.

But Iran - Damascus's closest ally - is vehemently opposed.

"We certainly do not agree agree with the suspension of any OIC member," Iranian Foreign Minister Ali Akbar Salehi said Monday. "We have to look for other ways, means and mechanisms for resolving conflicts and crises."

The Free Syrian Army claimed Monday it had shot down a warplane in the eastern province of Deir Ezzor and issued a video of a man it said was the captured pilot, a "staunch enemy of the revolution".

If confirmed, it would be the first time the rebels - who have been demanding anti-aircraft weapons to combat escalating attacks from the sky - succeeded in downing a government plane.

State media said the aircraft had crashed after suffering technical problems and that the pilot had ejected.

"We've seen a very troubling and despicable uptick in attacks from the air, perpetrated by the Syrian regime," Pentagon spokesman George Little told reporters.

Asked if Washington - which last week imposed new sanctions on Syria as it campaigns for Assad's ouster - was moving towards enforcing a possible no-fly zone, he said: "We plan for contingencies."

On the ground, the army shelled several rebel-held areas in Aleppo as fighting flared in the southwestern districts of Saif al-Dawla and Salaheddin, which was retaken by government forces last month, the Syrian Observatory for Human Rights said.

Pro-government daily Al-Watan said the capture of Salaheddin was but a "first step" in the retaking of all opposition areas in the city.

The metropolis - where communications have been cut for at least three days - is seen as pivotal to the outcome of the conflict, with some referring to it as Syria's Benghazi, the Libyan city at the heart of the revolt that toppled Moamer Kadhafi's regime.

In Damascus, security forces raided several districts after a major security operation on Monday, while several suburbs outside the capital were shelled, the Britain-based Observatory said.

Meanwhile, a reporter for Iran's Arabic language television network Al-Alam was abducted by rebels in the central city of Homs, the channel said, following a series of attacks on Syrian state media.