US President Barack Obama warned Syria of "enormous consequences" if it resorts to chemical weapons, as a female Japanese reporter was killed while covering clashes in the northern city of Aleppo.
Obama put President Bashar al-Assad's regime on notice on Monday that, while he had not ordered intervention "at this point", Washington was "monitoring the situation very carefully" and had put together a range of contingency plans.
The president said the United States would regard any recourse by Damascus to its deadly arsenal as crossing a "red line".
"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," he told reporters at a White House briefing.
Syria's admission in July that it has chemical weapons and could use them in case of any "external aggression" added a dangerous new dimension to a 17-month conflict which the new UN peace envoy already describes as civil war.
More than 130 people were killed 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.
In Syria's commercial capital Aleppo, nine civilians were reported killed late on Monday, including two women, a nine-year-old boy and a five-year-old girl.
The female Japanese journalist was killed after being caught in gunfire in Aleppo, the foreign ministry in Tokyo said on Tuesday.
The Britain-based Syrian Observatory for Human Rights said three other journalists — a Lebanese woman, an Arab male working for a US media outlet and a Turkish national — were missing while covering the Syria uprising.
The dead reporter was 45-year-old Mika Yamamoto, said an official in charge of Japanese nationals' safety abroad. A colleague travelling with her identified the body, the official said.
Yamamoto worked for the small Japan Press news agency and had also covered the war in Afghanistan and the Iraq conflict, according to the company's website.
She had been reporting on clashes between troops and rebel fighters in Suleiman al-Halabi, a district of Aleppo, the Observatory said.
Yamamoto was the fourth foreign reporter killed in Syria since March 2011.
French reporter Gilles Jacquier was killed on 11 January in the central city of Homs, where American journalist Marie Colvin and French photographer Remi Ochlik both perished on 22 February.
Lakhdar Brahimi, who has replaced former UN chief Kofi Annan as the international point man for Syria, warned on Sunday that it was now a matter of ending civil war rather than avoiding it.
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.
However, French President Francois Hollande bluntly told Brahimi at a meeting in Paris that there "cannot be a political solution without the departure of Bashar al-Assad".
The Syrian Observatory, meanwhile, reported fierce fighting across the country on Monday's second day of Eid al-Fitr, the holiday celebrated by Muslims to mark the end of the holy month of Ramadan.
The Observatory said troops backed by helicopters pounded several areas of Aleppo, including the local Baath party headquarters and a military tribunal in the northern city.
An AFP correspondent said warplanes and helicopters circled Aleppo all day, and reported heavy fighting in southwestern districts, where the regime's military operation against rebels is focused.
Aleppo has become the epicenter of the conflict since rebels seized large swathes of the city in a July offensive. Government officials have said its recapture will be the "mother of all battles".
The opposition Syrian National Council complained that government forces using combat helicopters, tanks and heavy artillery had carried out "savage" attacks on Herak, south of Damascus, warning that food and medicines were running out.
Fighting also flared in southern parts of Damascus as the army battled pockets of resistance despite claiming it retook most of the capital last month. In other parts of the country, 29 people were killed, half of them civilians.
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.