Riad Farid Hijab, Syria's prime minister who defected on Monday, has accused President Bashar al-Assad of carrying out "genocide" against his own people, prompting Washington to say the regime was "crumbling from within".
Patrick Ventrell, the US State Department's acting deputy spokesman, said the defections "indicate that the regime is crumbling and losing its grip on power".
Jay Carney, the White House spokesman, said: "This is a sign that Assad's grip on power is loosening.
"If he cannot maintain cohesion within his own inner circle, it reflects on his inability to maintain any following among the Syrian people that isn't brought about at the point of a gun."
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The White House repeated its call for Assad to step aside and end the violence gripping the country since an uprising againist Assad's rule which began in March last year.
Hijab announced on Monday through Muhammad el-Etri, his spokesman, that he was joining the opposition, after state television reported that he had been sacked.
The former prime minister said four decades of Assad family rule were collapsing.
Hijab, a leading Sunni Muslim in Assad's minority Alawite-dominated regime, became the highest-ranking official to flee Assad's regime in the nearly 17-month uprising when he crossed into Jordan on Sunday night.
He arrived in Jordan after being smuggled across the border, Jordanian authorities confirmed to Al Jazeera on Monday.
Hijab is to leave Jordan for Qatar within days, following the example of other high-profile defectors, Etri told the AFP news agency.
He was appointed prime minister on June 23 after parliamentary elections that the opposition termed a sham.
On Tuesday, Turkey's Anatolia news agency said that about 1,137 Syrians, including a general, have fled to Turkey overnight. The latest group brought the number of Syrian refugees in Turkey to nearly 50,000.
Fierce fighting in Aleppo
Meanwhile, the Syrian army continued to shell areas of the key battleground city of Aleppo on Tuesday, a human rights group said.
Rebel fighters fought with 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 Syrian Observatory for Human Rights, said.
The army also bombarded the Shaar, Sakhur and Qatarji districts in the east, it added.
Opposition activists of the Syrian Revolution General Commission said the army also used helicopter gunships to strafe the Hanano district of east Aleppo.
Fighting in the city of about 2.7 million people, Syria's largest, killed 46 people on Monday, the majority 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 which has been the scene of fierce fighting since July 20.
Pro-government daily Al-Watan said that the "terrorists", who claim to control around half of the city, have between 6,000 and 8,000 fighters.
AFP correspondents on the ground said that despite the fierce clashes in some neighbourhoods of the city, the army seemed to be biding its time before launching an all-out ground assault.
Lieutenant General Babacar Gaye, the head of the UN observer mission in Syria, issued an appeal on behalf of civilians on Monday, amid fears of a looming bloodbath.
"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.
The UN has said their observers in Syria have been forced to leave the city of Aleppo over security fears.
Meanwhile, Iran has expressed concern over the fate of more than 40 Iranians Tehran says are religious pilgrims kidnapped by rebels off a bus in Damascus while visiting Shia shrines.
The rebels say they suspect the captives are troops sent to aid Assad.
A rebel spokesman in the Damascus area said on Monday taht three of the Iranians had been killed by government shelling, and the rest would be executed if the shelling did not stop.
On Tuesday, Iran said the country's Foreign Minister Ali Akbar Salehi would travel to Turkey to discuss Syria and the situation of the Iranians.
The Iranian embassy said Salehi will meet Turkish Foreign Minister Ahmet Davutoglu in Ankara on Tuesday.
Al Jazeera's Nisreen El-Shamayleh, reporting from Amman, said: "The talks are going to focus on Syria, mainly the fate of 48 Iranian nationals who have been captured by rebels inside Damascus."
"Basically what we understand is that Iran has asked Turkey and Qatar, who have strong ties with Syrian opposition, to try to facilitate release of Iranian nationals," she said.
"Syrian opposition says they are not pilgrims but members of Iran’s revolutionary guard, they were trying to assist Syrian army in crushing dissent and kill opposition members."
Saeed Jalili, the secretary of Iran's Supreme National Security Council, is in Damascus for talks with the Syrian president, according to Iranian state TV.
"According to opposition groups, there is continuous co-ordination between Syrians and Iranians," Al Jazeera's El-Shamayleh said.
"Free Syrian Army says they have proof that Iranian security and military officials have been coming to Damascus meeting with Syrian officials giving them technical advice and tactical support to Syrian govt."
Source: Al Jazeera and agencies