Abdo Husameddine is the highest ranking civilian official to desert Mr Assad since the uprising against his authoritarian rule began a year ago.
That uprising is now slowly becoming a full-blown armed rebellion.
In a statement posted on YouTube, Mr Husameddine said: “I don’t wish to end my life servicing the crimes of this regime.
“This is why I have chosen to join the voice of righteousness, knowing that this regime will burn my home, persecute my family and come up with a lot of lies. I advise my colleagues who have been silent in the face of crimes for a year to abandon this sinking ship which is about to drown.”
It was not clear when or where the video was made and he did not disclose his current location. There was no comment from the regime in Damascus.
“I declare that I am joining the revolution of the dignified people,” Mr Husameddine said.
“You have inflicted on those you claim are your people a full year of sorrow and sadness, and denied them their basic rights to life and humanity and pushed the country to the edge of the abyss with your intransigence and detachment from reality. The economy of the country has reached near collapse.”
Mr Assad’s regime has suffered a steady stream of army defectors, who have joined a group of dissidents known as the Free Syrian Army, now numbering in the thousands. But civilian officials have remained largely loyal.
International pressure on Mr Assad reached a new peak on Wednesday when a senior US military commander said president Barak Obama was assessing options for military intervention in Syria.
Although there are widespread concerns that military action could cause a regional upheaval in the Middle East, General Martin Dempsey, chairman of the Joint Chiefs of Staff, told the US Senate foreign relations committee that Mr Obama has ordered a Pentagon review of options.
United Nations humanitarian chief, Valerie Amos, has also spoken after she visited the Baba Amr district of Homs following a deadly month-long siege. The military took control of Baba Amr on Thursday 1 March, but Amos was allowed in only this Wednesday, six days later.
She said yesterday she was struck by the devastation in the shattered now almost deserted. Activists claim Syrian forces conducted clean-up operations, including executions and arrests.
“The devastation is significant. That part of Homs is completely destroyed, and I am concerned to learn what happened to the people in that part of the city,” she said in Damascus, the regime’s stronghold.
“I have been struck by the difference between what I have seen here in Damascus and what I saw yesterday in Baba Amr,” she added.
But shortly after she spoke, Syrian security forces opened fire to disperse mourners in Mazzeh, a relatively prosperous neighbourhood of the capital. The crowd had gathered for the funeral of a soldier who was allegedly executed last month for refusing to obey orders to shoot at civilians in Homs.