He told a news conference in Jordan that the government's spirits were low after struggling for 17 months to crush the revolt against Assad's rule.

"Oh devoted revolutionaries, your revolution has become a model of effort and sacrifice for the sake of freedom and dignity. I assure you, from my experience and former position, that the regime is collapsing, spiritually and financially, as it escalates militarily," he said.

"It no longer controls more than 30 percent of Syrian territory... So let the shining revolution be completed by preserving the unity of the country."

Hijab, who like much of the opposition comes from Syria's Sunni Muslim majority was not part of Assad's inner circle. But as prime minister and the most senior civilian official to defect, his departure dealt a symbolic blow to the government, which is dominated by Assad's minority Alawite sect.

Syrian authorities said they had dismissed Hijab before he fled, but he told the news conference in Amman that he resigned and defected to the opposition, referring to the Assad government as an "enemy of God".

"It is my duty to wash my hands of this corrupt regime," Hijab said.

He thanked countries such as Saudi Arabia, Qatar and Turkey for their support and asked them to do more for the opposition.

"We call on them to continue their blessed efforts to support the people's revolt and bring them to the shores of certain victory," he said.