Syria has denounced international envoy Lakhdar Brahimi as "flagrantly biased", casting doubt on how long the UN-Arab League mediator can pursue his peace mission.
The Syrian foreign ministry was responding to remarks by Brahimi, a veteran Algerian diplomat, a day after he ruled out a role for President Bashar al-Assad in a transitional government and effectively called for the Baath Party leader to quit.
"In Syria ... what people are saying is that a family ruling for 40 years is a little bit too long," Brahimi told the BBC, referring to Assad, who inherited his post from his father Hafez, who seized power in 1970 and ruled for 30 years.
"President Assad could take the lead in responding to the aspiration of his people rather than resisting it," Brahimi said, hinting that Assad should go.
The foreign ministry in Damascus said it was very surprised at Brahimi's comments, which showed "he is flagrantly biased for those who are conspiring against Syria and its people".
Syria's Al Watan newspaper daily said Brahimi had removed his "mask of impartiality" to reveal his true face as a "a tool for the implementation of the policy of some Western countries".
Brahimi has had no more success than his predecessor Kofi Annan in his quest for a political solution to a 21-month-old conflict in which more than 60,000 people have been killed.
William Hague, UK foreign secretary, gave warning that violence in Syria might worsen, saying that the international community must "step up" its response if it does.
So far regional rivalries and divisions among big powers have blocked any concerted approach to the upheaval, one of the bloodiest to emerge from a series of revolts in the Arab world.
Russian and US diplomats, who back opposing sides of the war, are due to meet Brahimi in Geneva on Friday.
On Sunday Assad, making his first public speech in six months, offered no concessions and he said he would never talk to foes he branded terrorists and Western puppets.
In-depth coverage of escalating violence across Syria
As peace efforts floundered, rebels battled for a strategic air base for a second day, pursuing a civil war that had briefly receded for some Damascus residents who set aside their differences to play in a rare snowfall that blanketed the city.
For a few hours, people in the capital dropped their weapons for snowballs and traded hatred for giggles.
There was no respite on other battlefronts, with heavy fighting around the Taftanaz base in northwestern Syria, which rebels are trying to capture to extend their grip on Idlib province and weaken Assad's control of the skies.
Rebels assaulted the airport's main buildings and armoury using heavy guns, tanks and other weapons and appeared to have overrun half the area of the base, Rami Abdelrahman, director of the Syrian Observatory for Human Rights activist group, said.
Having tried to take the base for months, rebels have been bolstered by the recent arrival of Islamist fighters including the allegedly al-Qaeda-linked Nusra Front, he said.
Opposition forces have seized chunks of territory in northern Syria in recent months, but remain vulnerable to attack by the military's jets and helicopters.