ALEPPO, Syria: US President Barack Obama warned Syria Monday that any use of chemical weapons was a "red line" as fighting raged in the northern hub of Aleppo and the cradle of the uprising in the deep south.
"I have at this point not ordered military engagement," Obama told reporters at the White House.
But he added: "There would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons... That would change my calculations significantly."
Syria's admission in July that it has chemical weapons and could use them in case of any "external aggression" added a potentially dangerous new dimension to the conflict which the new international peace envoy describes as civil war.
At least 115 people were killed on Monday, including two children in shelling in Daraa, the birthplace of the revolution, a watchdog said, as the United Nations brought an end to its troubled observer mission.
Lakhdar Brahimi, who has replaced Kofi Annan as the international point man for Syria, had warned Sunday that it was now a matter of stopping rather than avoiding civil war after 17 months of bloodshed.
He also stoked opposition ire for refusing to say whether embattled President Bashar al-Assad should go.
"There are a lot of people who say that we must avoid civil war in Syria, me I believe that we are already there for some time now. What's necessary is to stop the civil war and that is not going to be easy," he said, describing it as the "cruellest kind of conflict".
But Syria -- which insists it is fighting an insurgency by "armed terrorist groups" backed by the West, Gulf states and Turkey -- reacted angrily.
"To speak of civil war in Syria contradicts reality and is found only in the heads of conspirators," the foreign ministry said in a statement carried by the state SANA news agency.
French President Francois Hollande bluntly told Brahimi at a meeting in Paris: there "cannot be a political solution without the departure of Bashar al-Assad."
The Syrian Observatory for Human Rights, meanwhile, reported fierce fighting across the country on the second day of Eid al-Fitr, the holiday celebrated by Muslims around the world to mark the end of the holy month of Ramadan.
The Observatory said troops backed by helicopters also pounded several areas of Aleppo, including the local Baath party headquarters and a military tribunal in Syria's second city.
An AFP correspondent said warplanes and helicopters were circling across the city throughout the day, and reported heavy fighting in southwestern districts where much of the regime's military operations against rebels are focused.
Epicentre of the conflict
"Where can we go, we don't have anywhere to go," lamented Tahani, 40, one of a group of women sheltering with children in the entrance of a house in the Ansari neighbourhood.
"In this situation having children is a curse. If it were just me, I wouldn't be afraid but I am always afraid for them."
Aleppo, the commercial capital, has emerged as the epicentre of the conflict since rebels seized large swathes of the city in an offensive launched on July 20. Government officials have said it will be the "mother of all battles."
The Observatory also said clashes erupted between rebels and government troops in Daraa after several areas were shelled, killing 16 people, including two children.
Government forces using combat helicopters, tanks and heavy artillery have also been carrying out "savage" attacks on Herak, the opposition Syrian National Council said, warning of a humanitarian catastrophe as supplies of food and medicines run out.
Fighting also flared in southern parts of Damascus as the army battles persistent pockets of resistance despite claiming it retook most of the capital last month. In the province, 29 people were killed, half of them civilians.
UN observers wound up their troubled mission on Sunday amid a failure by world powers to agree how to respond to Assad's crackdown and bring peace to the strategic Middle East state.
Its operations were largely suspended in June and observer numbers cut back in the face of the mounting violence, as both sides violated a ceasefire that was meant to have been the cornerstone of Annan's plan for peace.
A total of 23,000 people have now been killed since March last year, according to the Observatory, while the UN puts the death toll at around 17,000.
Amid speculation of further defections, Syria denied that Foreign Minister Walid Muallem had announced on Twitter he had replaced Vice President Faruq al-Shara, who the opposition claimed had tried to defect.
SANA said the information was "wrong" and that Muallem did not have a Twitter account.
Several high profile officials, including former prime minister Riad Hijab, have abandoned the regime in recent months.
The conflict has raised fears of a spillover into neighbouring countries, which are sheltering several hundred thousand refugees who have fled the fighting.
Turkey warned it can handle no more than 100,000 refugees and has proposed setting up a UN buffer zone inside Syria to shelter them.
The United Nations has warned of a humanitarian crisis with more than one million people also displaced inside Syria and up to 2.5 million in need of aid.