A Syrian deputy oil minister says he is resigning to join the revolt against the government.
Abdo Hussameddin, 58, announced his defection in a video posted on YouTube.
He is the highest level political figure to abandon the government of President Bashar al-Assad since the uprising erupted a year ago.
Earlier, after a visit to the city of Homs, the UN humanitarian chief said some areas had been "devastated" in the offensive by Syrian government troops.
Valerie Amos said the bombed-out Baba Amr district felt like it had been closed down entirely. The government retook the district last week after fierce shelling.
Activists say troops committed massacres since they went in. Damascus blames rebels for many deaths.
'Driven by barbarism'
Abdo Hussameddin - who is one of two deputy oil ministers - posted his video on YouTube late on Wednesday.
Wearing a smart jacket, collar and tie, and sitting in a high-backed armchair, he read out a four-minute ringing denunciation of the regime he said he had served in one capacity or another for the past 33 years.
"I, Abdo Hussameddin, deputy oil and mineral wealth minister in Syria, announce my defection from the regime, resignation from my position and withdrawal from the Baath Party.
"I am joining the revolution of the people who reject injustice and the brutal campaign of the regime."
Mr Hussameddin - deputy oil minister since August 2009 - added: "I tell the regime, which claims to own the country, you have nothing but the footprint of the tank driven by your barbarism to kill innocent people."
He said he was stepping aside although he knew that his house would be burnt and his family persecuted by the regime.
An activist who shot the video and posted it on YouTube told the AFP news agency in Beirut that the opposition had helped to arrange the resignation.
The Syrian government has not publicly commented on Mr Hussameddin's announcement.
Observers say public defections have been rare among civilian officials of the Syrian state, which is controlled by President Assad's minority Alawite sect.
However, there have been high-profile defections from the military, including Gen Mustapha al-Sheikh who fled to Turkey earlier this year. Also thousands of chiefly Sunni soldiers and conscripts are reported to have deserted since the start of the uprising.
A spokeswoman from the opposition National Transitional Council of Syria said she believed many more cabinet members and their deputies were prepared to defect.
The BBC's Jim Muir in Beirut says the deputy minister's resignation would appear to signal growing strains within the regime, as the violence intensifies, and the economy comes under increasing stress because of sanctions.
Violence continued across Syria on Wednesday. Opposition groups said 39 people were killed - 26 in Homs, six in Idlib, three in Deraa and two each in suburbs of Damascus and Aleppo.
The UN says more than 7,500 people have died as a result of the violence in Syria over the past 12 months.
International media organisations are heavily restricted in Syria, making it impossible to verify the claims of either side.
'No to force'
In further diplomatic efforts to halt the violence, special envoy Kofi Annan is due to meet representatives of both sides in Damascus at the weekend.
Speaking after talks in Cairo on Thursday, Mr Annan - joint envoy for the UN and Arab League - rejected any military intervention in Syria.
"I hope no-one is thinking very seriously of using force in this situation. I believe further militarisation will make the situation worse," he said after meeting Arab League Secretary General Nabil al-Arabi.
Separately, Beijing announced on Thursday that its envoy had talks in Syria this week with representatives of the government and the opposition.
China's foreign ministry said envoy Li Huaxin met Syrian Foreign Minister Walid al-Moallem and his deputy during a two-day visit.
Observers say Mr Li's visit is Beijing's latest attempt to counter charges by Western and Arab leaders that by vetoing two previous UN resolutions, China and Russia have aided the growing violence by Syrian government forces.
On Wednesday, US Defence Secretary Leon Panetta said no options - including military action - had been ruled out to end the violence in Syria.
However Mr Panetta - appearing before the Senate Armed Services Committee - stressed that the US still preferred a diplomatic solution.
Washington is currently trying to draft a new UN Security Council resolution to try to end the violence.
Russia and China said the earlier motions were unbalanced.