The Syrian army shelled rebel areas of key battleground city Aleppo on Tuesday a day after the shock announcement by now ex-prime minister Riad Hijab that he had joined the opposition.
Hijab was in Jordan firming up his plans after he slipped across the border on Sunday night in the highest-ranking defection of the nearly 17-month uprising, which Washington said showed President Bashar al-Assad's regime was crumbling.
Opposition spokesmen said Hijab would head to staunch opposition supporter Qatar and could arrive later on Tuesday.
Key Assad ally Iran sent top envoys to both Syria and Turkey as it found itself increasingly embroiled in the intensifying conflict which a human rights watchdog said killed at least 226 people on Monday, 147 of them civilians.
Iran is deeply concerned about the fate of 48 of its citizens abducted on the weekend by a rebel group. Tehran denies the group's claims the hostages include members of its elite Revolutionary Guards, and insists all are pilgrims.
Fierce fighting rocked the heart of commercial capital Aleppo early on Tuesday, the Syrian Observatory for Human Rights said, but AFP correspondents on the ground said it did not appear to mark the launch of the all-out ground offensive the army has been threatening since Sunday.
Rebels battled troops in the Bab Antakya, Aziziyeh, Bab Janin and Sabaa Bahrat areas of the city centre and near the Palace of Justice in the west, the Britain-based watchdog said.
The army also bombarded the Shaar, Sakhur and Qatarji districts in the east, it added.
Fighting in the city killed 46 people on Monday, most of them civilians, the Observatory said.
A senior security official said on Sunday that the army had completed its deployment of reinforcements to Aleppo, building up a force of about 20 000 soldiers, ready for a decisive showdown in the city that has seen fierce fighting since July 20.
Pro-government newspaper Al-Watan reported on Tuesday that the army's operations so far had been “preliminary” but said they had already killed dozens of rebels.
The head of the UN observer mission in Syria, Lieutenant General Babacar Gaye, issued an appeal on Monday on behalf of civilians in the city of about 2.7 million people.
“I urge the parties to protect civilians and respect their obligations under international humanitarian law. Civilians must not be subjected to shelling and use of heavy weapons,” he said.
Approximately 20 unarmed UN observers were moved from Aleppo back to the mission's Damascus headquarters at the weekend because of worsening security, a UN spokeswoman said.
Assad's government put on a brave face after Hijab's defection but Washington said it showed it had lost control.
Information Minister Omran al-Zohbi said defections at whatever level would have no impact, implicitly acknowledging the premier'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.
In Washington, National Security Council spokesman Tommy Vietor said Hijab's defection was “just the latest indication that Assad has lost control of Syria and that the momentum is with the opposition forces and the Syrian people”.
He said the quickening spate of defections “indicate that the regime is crumbling and losing its grip on power.”
Saeed Jalili, a top aide to Iran's supreme leader Ayatollah Ali Khamenei, flew to Damascus for talks with Assad.
“The Islamic Republic of Iran believes in national dialogue between all domestic groups to be the solution, and believes foreign solutions are not helpful,” Iran's state media quoted him as saying on his arrival.
“We hope to take an effective step in regards to this new direction.”
Iran has been increasingly critical of support by the United States, Turkey, Saudi Arabia and Qatar for the rebels in Syria.
It is also increasingly concerned about the fate of its 48 hostages following an unconfirmed report by a rebel group on its Facebook page that three of them had been killed in shelling by Assad's forces on Monday.
Iranian Foreign Minister Ali Akbar Salehi was due in Turkey later on Tuesday on another hastily arranged visit to demand Ankara's assistance in securing the release of the hostages.
“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 in a statement carried by the official IRNA news agency.
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. - Sapa-AFP