The newly appointed head of the opposition Syrian National Council (SNC) has called on government officials in Damascus to defect.
Speaking in Istanbul, Abdelbaset Sayda reiterated the group's rejection of foreign intervention, unless it was sanctioned by the United Nations.
Meanwhile, violence has intensified in the province of Homs, with reports of blasts and machine-gun fire on Monday.
At least 35 people were killed there in bombardments on Sunday, activists said.
The BBC's Paul Danahar, who is in Homs city, says despite the fighting the city centre is busy, although the road to the old town, where much of the clashes are taking place, has been blocked by the army
Abdelbaset Sayda, a Kurd, replaced Burhan Ghalioun, who had held the post since the group was established last September.
He told the BBC World Service the SNC was not calling for "a foreign war or intervention. Rather it is the regime that is pushing our country in this direction, that is waging unjust war on the nation and the people".
"Our intentions are peaceful, but this bestial regime is bent on extermination and a scorched earth policy."
Mr Sayda, who lives in exile in Sweden, said the situation in Syria was "entering a sensitive phase", and that President Bashar al-Assad's regime was "on its last legs", AFP reported.
"The multiplying massacres and shellings show that it is struggling," he told the news agency.
He also urged "all officials in the regime and in the institutions to defect".
The BBC's Jim Muir in Beirut in neighbouring Lebanon says there have been some defections going on all along but very few by way of whole army units or politically from the regime.
On Sunday an air defence base north of Homs - a unit of about 100 people - defected en masse, before the military sent in helicopters to destroy the base to prevent missiles falling into rebel hands, according to opposition activists.
Since its inception, the SNC has been plagued by divisions and complaints from activists that it is ineffectual.
There have also been accusations that the council, which is an umbrella organisation for opposition groups, is dominated by Islamists and not inclusive enough.
However, Mr Sayda said any post-Assad Syria would reach out to its patchwork of ethnic groups.
"We would like to reassure all sects and groups, especially Alawites and Christians, that the future of Syria will be for all of us," he said.
At least 32 civilians were killed across Syria in continuing violence on Sunday, opposition activists said, with at least 20 deaths in Homs province alone.
They said the army used artillery, mortars and rockets against opposition positions in the city of Homs, and in the towns of Qusair, Talbiseh and Rastan, Reuters news agency reported.
Security forces also targeted rebels in suburbs of the capital, Damascus, the agency quoted opposition sources as saying.
Sixteen soldiers were killed by rebels, according to the UK-based Syrian Observatory for Human Rights. These reports cannot be independently verified, as Syria severely restricts journalists' freedom movement.
The UN says that at least 10,000 people have been killed since the uprising against Mr Assad began in March 2011.