Peace envoy Lakhdar Brahimi held "constructive" talks in Syria with President Bashar al-Assad, as Washington warned that his regime's days are numbered.
As jihadists seized an area populated by the embattled leader's Alawite community, the opposition National Coalition accused Damascus of committing a "massacre" of dozens of civilians in the bombing of a bakery.
The United States condemned the "vicious" attack in which at least 60 people are reported to have been killed in a regime air strike on a bakery in the town of Halfaya, in the central province of Hama on Sunday.
"Brutal attacks such as these show that this regime has no future in Syria," acting State Department spokesman Patrick Ventrell said in a statement.
"Those that commit atrocities will be held accountable. The United States calls on all parties that continue to assist the regime in executing its war against the Syrian people to end their support," he added.
As violence raged in flashpoints across Syria, some 1,000 people attended Christmas mass in Damascus, praying for peace to return nearly two years into an uprising that has killed tens of thousands.
Heba Shawi said she hoped "the smile comes back to children's faces" during the festivities, which other church-goers admitted would be much more low key than usual.
Hours earlier, Brahimi, the UN and Arab League envoy to Syria, met with Assad, who described the talks as "friendly and constructive".
"I had the honour to meet the president and as usual we exchanged views on the many steps to be taken in the future," Brahimi said, while labelling the crisis as "worrying" given the scale of the bloodshed.
More than 44,000 people are estimated to have been killed since the eruption in March 2011 of the uprising that morphed into an armed insurgency when the Assad regime unleashed a brutal crackdown on dissent.
On Monday alone, at least 119 people were killed nationwide, including 38 civilians, said the Syrian Observatory for Human Rights.
Brahimi, who last visited Syria on October 19, expressed hope "all parties are in favour of a solution that draws Syrian people together".
"Assad expressed his views on the situation and I told him about my meetings with leaders in the region and outside," said the veteran Algerian diplomat who took over the position from former UN chief Kofi Annan.
Assad said his "government is committed to ensure the success of all efforts aimed at protecting the sovereignty and independence of the country", state television reported.
The official SANA news agency blamed the bakery killings on an "armed terrorist group" -- the regime term for rebels -- saying "many women and children" had died.
The National Coalition, recognised by many countries and groupings as the legitimate representative of Syrians, blamed Assad's regime for the "massacre" in Halfaya, saying it "targeted children, women and men who went out to get their scarce daily bread ration".
Meanwhile in Hama, the Observatory said the Al-Nusra Front and other jihadist groups on Monday overran large parts of the village of Maan populated by Alawites, the offshoot of Shiite Islam to which Assad belongs.
Rebels last week launched an all-out assault on army positions across Hama, home to a patchwork of religious communities, the Observatory said.
Activists accused Assad's regime of unleashing killer gas bombs in the central city of Homs.
The Observatory said six rebels died in Homs on Sunday night after inhaling "odourless gas and white smoke" emanating from bombs deployed by regime forces in clashes with rebels.
"These are not chemical weapons, but we do not know whether they are internationally prohibited," Observatory director Rami Abdel Rahman told AFP.
Russia, one of the few staunch allies of Syria, downplayed fears of chemical weapons being deployed.
"I do not believe Syria would use chemical weapons," Foreign Minister Sergei Lavrov told English-language television channel RT. "It would be a political suicide for the government if it does."
Meanwhile, rights watchdog Amnesty International condemned the regime's transfer of civilians to military courts, and urged action to ensure Syria's courts meet international fair trial standards.