Syria dismissed reports that President Bashar al-Assad's deputy had defected and its forces pursued an offensive against rebels, bombarding parts of Aleppo in the north and attacking an insurgent-held town in the oil-producing east.
Vice-President Farouq al-Shara "never thought for a moment about leaving the country", said a statement from his office broadcast on state television on Saturday in response to reports that the veteran Baath Party loyalist had tried to defect to Jordan.
Mr. Assad, battling a 17-month-old rebellion led by Syria's Sunni Muslim majority, has been abandoned by a number of senior officials, including prime minister Riyadh Hijab two weeks ago.
Mr. Shara, whose cousin - an intelligence officer - announced his own defection on Thursday, is a Sunni Muslim from Deraa province where the revolt began against Mr. Assad, a member of the minority Alawite sect that is an offshoot of Shi'ite Islam.
The 73-year-old former foreign minister kept a low profile as the rebellion mushroomed but appeared in public last month at a state funeral for three of Mr. Assad's top security officials killed in a bomb attack in Damascus.
The statement said he had worked since the start of the uprising to find a peaceful, political solution and welcomed the appointment of Algerian diplomat Lakhdar Brahimi as a new international mediator for Syria.
Mr. Brahimi, who hesitated for days before accepting a job that France's UN envoy Gerard Araud called an "impossible mission," will replace former UN Secretary-General Kofi Annan, who is leaving at the end of the month.
Mr. Annan's six-point plan to stop the violence and advance towards political negotiations was based on an April ceasefire agreement which never took hold. The conflict has deepened since then with both sides stepping up attacks.
Mr. Assad's forces have resorted increasingly to air power to hold back lightly armed insurgents in the capital Damascus and Aleppo, a northern commercial hub. More than 18,000 people have died in the bloodshed and about 170,000 have fled the country, according to the United Nations.
The Syrian Observatory for Human Rights said the army bombarded neighbourhoods in Aleppo. Rebels hold several districts in the country's largest city and have tried to push back an army counter-offensive.
State television said soldiers "cleared terrorists and mercenaries" - terms used by authorities to describe Mr. Assad's armed opponents - from the western district of Saif al-Dawla, where some of the heaviest fighting has taken place.
Internet footage which activists said was filmed in Saif al-Dawla on Saturday showed a plane making a low pass over buildings and dropping two bombs.
The Observatory also said at least 20 armoured vehicles moved into the eastern town of Mayadeen in Deir al-Zor province, where Syria's 200,000 barrels per day of oil are produced.
More than 130 people were killed in Syria on Saturday, it said, including 15 in Deir al-Zor.
In the town of Tel, north of Damascus, local activists said the bodies of 40 people killed by bombardment were gathered together for a joint burial. A picture showed what appeared to be several corpses wrapped in colourful blankets on a street.
Mr. Brahimi will have a new title, Joint Special Representative for Syria. Diplomats said this was to distance him from Mr. Annan, who complained that his peaceful transition plan was crippled by divisions between Western powers - who want Mr. Assad out - and Russia, his most important ally - in the UN Security Council.
Describing the situation in Syria as "absolutely terrible", Mr. Brahimi told Reuters he urgently needed to clarify what support the United Nations can give him and said it was too soon to say whether Assad should step down - in contrast to Mr. Annan who said it was clear the Syrian leader "must leave office".
The last UN observers who deployed in Syria four months ago to monitor Mr. Annan's failed ceasefire will leave after midnight on Sunday, when their mandate expires.
They will leave a "liaison office" open in Damascus after their departure, though its size and role have not been finalised, a UN spokeswoman said.