U.S. 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, according to AFP.

Obama put President Bashar al-Assad's regime on notice Monday, Aug 20, 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 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 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.