Former Syrian PM Riad Hijab, who defected to Jordan last week, has said the Syrian regime is collapsing "morally, financially and militarily".
Speaking in the Jordanian capital, Amman, he said the regime did not control more than 30% of the territory.
He called on the opposition abroad to unite and on the Syrian army to stand alongside its people.
UN humanitarian chief Valerie Amos is in Syria to assess ways to increase the flow of emergency aid there.
Mr Hijab added he was joining the rebel side and urged other political and military leaders to break away from President Bashar al-Assad's regime.
He said he could no longer support President Assad because he had no ability to halt the fighting.
This is the first time Mr Hijab has spoken publicly since fleeing to Jordan with his family last week.
He is the highest-ranking political figure to defect from the Assad regime.
Baroness Amos is expected to ask for more foreign aid workers to be given visas, as the Syrian Arab Red Crescent struggles to distribute food to those who need it.
She will then go to Lebanon, to see what help refugees there need.
Meanwhile, an aide to President Assad has flown to Beijing for talks on the crisis with Chinese officials.
A Chinese foreign ministry spokesman said it was also considering an invitation to members of the Syrian opposition.
Beijing has opposed recent UN resolutions on Syria, but backs a ceasefire between the warring parties as well as political dialogue.
On Monday, the secretary general of the Organisation of Islamic Co-operation (OIC) said foreign ministers preparing for a summit in Mecca had agreed to recommend the suspension of Syria.
However, Iranian Foreign Minister Ali Akbar Salehi insisted his country was opposed to such a step, saying: "We have to look for other ways, means and mechanisms for resolving conflicts and crises."
'Scaling up efforts'
Baroness Amos, the UN under-secretary general and emergency relief co-ordinator, arrived in Damascus on Tuesday in a land convoy from Lebanon, a spokesman said.
She is scheduled to meet senior Syrian officials, including the newly appointed Prime Minister, Wail al-Halqi, Foreign Minister Walid Muallem and his deputy, Faisal Miqdad. Later, she will hold talks with the Syrian Arab Red Crescent.
The UN said she was expected to discuss ways of "urgently scaling up relief efforts and reducing the suffering of civilians caught up in the fighting", which has intensified in recent weeks and spread to the two biggest cities, Damascus and Aleppo.
The three-day visit to the region "aims to draw attention to the deteriorating humanitarian situation in Syria and the impact of the conflict on people either remaining in Syria and who have fled to other countries", the UN added.
Two million people are now estimated to have been affected by the crisis and over one million have been internally displaced. More than 140,000 people have fled the violence and crossed into Lebanon, Jordan, Turkey and Iraq.
In Lebanon, Baroness Amos is set to meet families who have fled from Syria and to discuss with the government and humanitarian agencies how best to support the refugees.
'Downed fighter jet'
On Tuesday, fresh fighting was reported by activists in Aleppo, Syria's second city, with the army shelling several rebel-held areas.
"Fighting broke out between government forces and rebel fighters in the districts of Saif al-Dawla and Salah al-Din alongside explosions in the area," the UK-based Syrian Observatory for Human Rights said.
In Damascus, people were reportedly fleeing the central district of Qabun, fearing a military offensive as security forces raided two other districts.
On Monday, rebel fighters said they had shot down a government fighter plane near the border with Iraq. Footage of the attack posted online showed a jet bursting into flames amid the sound of heavy gunfire.
Military experts said that the plane shown in the footage is a MiG-23BN, and that its markings and camouflage were consistent with that of one belonging to the Syrian air force.
The rebels also said they had captured the pilot and posted a video showing them questioning a man.
Syrian state television has confirmed that a plane has crashed, but said it was due to technical problems.
Syrian human rights and opposition activists say at least 21,000 people have been killed since pro-democracy protests erupted in Syria in March 2011.