Syrian Prime Minister Riad Hijab is joining the rebels in protest at the "genocide" President Bashar al-Assad is carrying out against his own people, his spokesman said, while state television reports Mr Hijab was sacked.
"I announce my defection today (Monday) from the regime of killing and terror, and I join the ranks of the revolt," he said in statement read by his spokesman Mohammed al-Otri on Al-Jazeera news channel from Amman.
He said his defection comes at a time "when Syria is passing through the most difficult war crimes, genocide, and barbaric killings and massacres against unarmed citizens."
Otri said the premier was in a "safe haven" with his family.
State television reported that Deputy Prime Minister and Local Government Minister Omar Ghalawanji had been appointed caretaker premier.
"Prime Minister Riad Hijab has been dismissed," it said in a terse report.
According to the state-owned Tishrin newspaper, Hijab presided over two meetings at the local government ministry on Sunday to discuss "measures to redevelop areas that have been cleansed of armed terrorists."
If confirmed, Hijab's defection would be the highest-ranking of the 17-month uprising, and a new blow to President Bashar al-Assad, who has already seen no fewer than 31 of his generals cross the border into Turkey to join the rebellion and a growing number of his ambassadors break ranks.
Hijab was a leading Sunni Muslim in Assad's minority Alawite-dominated government. His home province of Deir Ezzor in the northeast has been one of the key battlegrounds of the conflict and seen a mounting death toll from operations by the army in recent weeks.
The 46-year-old was only appointed on June 6 following a widely boycotted May 7 parliamentary election that was hailed as a centrepiece of reform by the Assad regime but dismissed as a farce by Arab and Western governments.
An agricultural engineer by training, Mr Hijab was agriculture minister under his predecessor government Adel Safar who was appointed in April 2011, shortly after the outbreak of the uprising.
"Riad Hijab has defected from the regime," the director of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
Abdel Rahman said there were conflicting reports on Hijab's current whereabouts.
"Some sources say he has arrived in Jordan, others say he was arrested before making his escape," said Abdel Rahman.
Reports of his defection emerged as the army readied a major ground assault against rebels in commercial capital Aleppo, who say they control half of the city of some 2.7 million people.
They also came as a bomb blast rocked Syrian state television headquarters in the heart of Damascus wounding several people just two days after the army said it had seized the last rebel-held area of the capital.
The morning bombing struck management offices on the third floor of the television building in the heavily protected Omayyad district of the capital.
"It is clear that the blast was caused by an explosive device," said Information Minister Omran al-Zoabi. "Several of our colleagues were injured, but there were no serious injuries, and no dead."
Pro-government television channel Al-Ikhbariya, which was itself the target of a deadly attack claimed by the rebel Free Syrian Army in June, broadcast footage of Zoabi inspecting the building's third floor.
The walls were visibly damaged, water pipes broken, and electric cables hung down from the ceiling. Blood could also be seen on some of the furniture. The broadcaster showed volunteers evacuating a wounded man.
"Syria's television is being targeted because of its bravery," Zoabi said. "But nothing will stop the voice of Syria."
On June 27, gunmen armed with explosives attacked the Al-Ikhbariya offices outside Damascus killing three journalists and four security guards.
On Saturday, rebel fighters attacked the state television building in Syria's second city, Aleppo.
The same day, the Syrian Observatory reported that state television presenter Mohammed al-Saeed had been executed following an abduction claimed by the Al-Qaeda-linked Al-Nusra Front.
Posted on a forum featuring the Al-Qaeda flag, Al-Nusra's statement showed a photograph of Saeed looking frightened with his back against a wall in an unknown location.
"May this be a lesson to all those who support the regime," it said.
In Aleppo, the army bombarded a string of rebel neighbourhoods after government security officials said that troops had completed their build-up and that a 20,000-strong force was poised for a ground assault.
A rebel commander was killed in the Salaheddin district in the southwest, and troops shelled the Palace of Justice, as well as the Marjeh and Shaar districts, the Syrian Observatory said.
A total of nine people were killed in Aleppo early on Monday, among them eight civilians, the watchdog.
A senior security official said on Sunday that the army had completed its deployment of reinforcements to Aleppo, ready for a decisive showdown.
"All the reinforcements have arrived and they are surrounding the city," the official said. "The army is ready to launch its offensive, but is awaiting orders."
Elsewhere in Syria, the Observatory reported another 19 deaths early Monday -- 13 civilians and six rebels.