Syrian regime warplanes pounded rebel positions in second city Aleppo on Sunday ahead of a threatened ground assault by more than 20,000 troops assembled around the commercial capital.

Iran appealed for help from governments with ties to the Syrian opposition in securing the release of 48 of its nationals seized from a bus in Damascus, as an Arabic news channel aired footage it said was of the Iranians in the hands of rebel captors who charged that their hostages were Revolutionary Guards.

A senior security official said Syria's army had completed its deployment of reinforcements to Aleppo, ready for a decisive showdown.

"The war is likely to be long, because there will have to be street battles in order to get rid of the terrorists," the source told AFP, declining to be named.

"All the reinforcements have arrived and they are surrounding the city," he said. "The army is ready to launch its offensive, but is awaiting orders."

Clashes and shelling killed at least two rebels and three civilians in Aleppo, the Syrian Observatory for Human Rights said, contributing to Sunday's nationwide death toll of 47 -- 31 civilians, 10 soldiers and six rebels.

Troops shelled the city's rebel-held Salaheddin district in the southwest and clashes erupted in the neighbourhoods of Sukari, Hamdaniyeh and Ansari, it said.

Observatory director Rami Abdel Rahman later told AFP that "fighter jets are being used to pound Shaar and Sakhur" in the northern city.

The opposition Syrian National Council charged that the bombardment of Aleppo, the scene of fierce fighting since July 20, was hitting some historical sites.

"After failing to subdue (rebels) in Aleppo... the Syrian regime's gangs have started to target government institutions and buildings," the exiled group said. "Some of them have historical and archaeological value."

Aleppo has a raft of renowned sites, including its 13th century citadel. The UN Educational, Scientific and Cultural Organisation named the Ancient City a World Heritage Site in 1986.

The SNC accused the army of shelling Aleppo's television building.

"The criminal regime does not hesitate to shell these institutions," the group said. "The rebels were forced to move away from the television building in order to protect the Syrian people's property and heritage."

Rebels tried to storm the state television building on Saturday before being driven back by shelling, the Observatory said.

-- 'Just the appetiser' --

A senior government security figure warned at the weekend that "the battle for Aleppo has not yet begun, and what is happening now is just the appetiser... The main course will come later."

The official said at least 20,000 troops were on the ground. "The other side are also sending reinforcements," he added of the rebels, who claim to have seized half the city.

The pro-government Al-Watan newspaper said the army had killed "hundreds of terrorists" in Aleppo but that 6,000 to 8,000 remained.

The army said on Saturday it had seized Tadamun, the last rebel-held district of Damascus, after heavy fighting, and authorities took journalists on an escorted tour.

"We have cleansed all the districts of Damascus, from Al-Midan to Mazzeh, from Al-Hajar Al-Aswad to Qadam... to Tadamun," an officer told reporters on the tour.

The fighting has displaced tens of thousands, including some 600 Palestinian families who arrived in Lebanon in the past three days, most escaping violence at the Yarmuk refugee camp in Damascus, a Palestinian official in Lebanon said.

Iran appealed to Qatar and Turkey for help in securing the release of 48 nationals it says were seized while on pilgrimage to a shrine in Damascus.

Al-Arabiya television aired footage on Sunday which it said was of the Iranians in the captivity of rebel Free Syrian Army fighters who charged that their hostages were elite Revolutionary Guards.

Fighters of the FSA's Al-Baraa Brigade had "captured 48 of the shabiha (pro-regime militiamen) of Iran who were on a reconnaissance mission in Damascus," said a man dressed as a rebel officer shown in the footage.

"During the investigation, we found that some of them were officers in the Revolutionary Guards," he said, showing documents taken from one of the men, who appeared in the background.

Tehran, Damascus's key regional ally, has repeatedly denied sending any military units to Syria.

In the latest reported high-level defections, three intelligence officers abandoned the Assad regime and sought refuge in neighbouring Jordan, a rebel spokesman told AFP.

Among them were Colonel Yarab al-Shara and his brother Mohammed Kanaan al-Shara who are from the same clan as Syria's Sunni Muslim vice president, Faruq al-Shara.

The report came after Anatolia news agency said General Muhammed Ahmed Faris, a military aviator who became the first Syrian in space, fled to Turkey on Sunday.

On the diplomatic front, US Secretary of State Hillary Clinton is to travel to Turkey next Saturday for talks on the conflict in Syria, the State Department said.

burs/dv/srm