DAMASCUS - Syrian President Bashar al-Assad appeared on television on Tuesday for the first time in more than two weeks, meeting a top envoy from closest ally Iran as his troops engaged rebels in key battleground city Aleppo.

Tehran, which has voiced growing criticism of support by the United States, Turkey, Saudi Arabia and Qatar for the Syria rebels, also sent its foreign minister to Ankara and a stern letter to Washington holding them responsible for the fate of 48 Iranians kidnapped by the rebels.

Saeed Jalili, a top aide to Iran's supreme leader Ayatollah Ali Khamenei, went straight into his meeting with Assad on arriving from neighbouring Lebanon a day after Syria's prime minister abandoned the regime.

"Assad and Jalili discussed bilateral relations between Syria and the Islamic Republic of Iran as well as the situation in the region," state television reported.

The last time footage of Assad was screened was when he received new armed forces chief General Ali Ayyub on July 22, four days after a bombing claimed by the rebel Free Syrian Army killed four top security chiefs.

Iranian state media quoted Jalili as saying Tehran "believes in national dialogue between all domestic groups to be the solution, and believes foreign solutions are not helpful."

"We hope to take an effective step in regards to this new direction."

In Beirut on Monday, Jalili issued a veiled warning to countries backing the rebels.

"Those who believe that, by developing insecurity in the countries of the region by sending arms and exporting terrorism, they are buying security for themselves are wrong," he told Lebanese Foreign Minister Adnan Mansour, Iran's official IRNA news agency said.

In commercial capital Aleppo, clashes between troops and rebels rocked several areas of the city centre early on Tuesday, while the army also shelled rebel-held areas in the east, the Syrian Observatory for Human Rights said.

The fighting in Aleppo killed at least seven people, the Britain-based watchdog said, adding that the nationwide toll was 51.

At least 265 people were killed on Monday, 182 of them civilians, making it one of the deadliest days of the uprising, the Observatory said. Of those, 57 were in Aleppo.

Troops fought rebels in the Bab Antakya, Aziziyeh, Bab Janin and Sabaa Bahrat areas of central Aleppo and near the Palace of Justice in the west, it said.

Fighting also broke out for the first time in the Ashrafiyeh district in the northwest when rebels attacked a military post, Observatory director Rami Abdel Rahman said.

Aleppo has been bracing for a threatened major ground offensive by the army against the rebels, who say they control around half of the city.

A senior security official said on Sunday the army had completed the build-up of some 20,000 troops in readiness for a decisive showdown in the battle underway since July 20.

The head of the UN observer mission in Syria, Lieutenant General Babacar Gaye, expressed concern for civilians trapped in the city of some 2.7 million people.

"I urge the parties to protect civilians and respect their obligations under international humanitarian law," Gaye said on Monday. "Civilians must not be subjected to shelling and use of heavy weapons."

The UN mission pulled out its own 20-strong team from Aleppo at the weekend in the face of the worsening violence, a UN spokeswoman said.

Defected ex-prime minister Riad Hijab was in neighbouring Jordan firming up his plans after his shock defection to the opposition, which Washington said showed Assad's regime was crumbling.

US National Security Council spokesman Tommy Vietor called Hijab's defection "just the latest indication that Assad has lost control of Syria and that the momentum is with the opposition forces and the Syrian people."

Assad's government put on a brave face.

Information Minister Omran al-Zohbi said defections at whatever level would have no impact, implicitly acknowledging Hijab's flight.

"Syria is a state of institutions and the defection of individuals, whatever their rank, does not change the policy of the state," Zohbi told the official SANA news agency.

Iranian Foreign Minister Ali Akbar Salehi headed to Turkey to demand Ankara's assistance in securing the release of the Iranian hostages amid growing concern for their fate. That followed an unconfirmed report on a rebel group's Facebook page that three of them had been killed in shelling by Assad's forces on Monday.

"Considering that the (rebel) Free Syrian Army -- which claims to have abducted the Iranian pilgrims -- is backed by Turkey, the visit by the foreign minister aims to warn and remind the Ankara government of its responsibilities in this matter," the foreign ministry said.

Tehran delivered a similar message to Washington in a letter transmitted through the US interests section of the Swiss embassy.

"Because of the United States' manifest support of terrorist groups and the dispatch of weapons to Syria, the United States is responsible for the lives of the 48 Iranian pilgrims abducted in Damascus," Deputy Foreign Minister Hossein Amir Abdollahian quoted the letter as saying.

On the humanitarian front, more than 22,000 Iraqis have fled Syria in less than three weeks, while 12,600 Syrians have done so since the beginning of the year, the UNHCR representative in Baghdad said.

In Geneva, the World Health Organisation said Syrians urgently need life-saving medicines, and the World Food Programme said 1.5 million people in rural areas will need food aid in the next three to six months.