Barack Obama has warned Syria's Assad regime of "enormous consequences" if it resorts to chemical weapons in its 17-month conflict with opposition forces.
The US president said that while he had not ordered intervention "at this point", Washington was "monitoring the situation very carefully".
Speaking as it emerged that a Japanese journalist had been killed in heavy fighting in Syria's biggest city, Aleppo, Mr Obama said the use of chemical weapons would cross a "red line".
"There would be enormous consequences if we start seeing movement on the chemical weapons front or the use of chemical weapons," he said at a White House briefing.
"That would change my calculations significantly."
Syria said in July that it could use chemical weapons if faced with any "external aggression".
More than 130 people were killed in a day of fierce fighting on Monday, according to the British-based Syrian Observatory for Human Rights, including two children in shelling in Deraa.
In Aleppo, nine civilians were reported killed, including two women and two children.
The female Japanese journalist, named as 45-year-old Mika Yamamoto, was killed after being caught in gunfire.
Fellow Japan Press reporter Kazutaka Sato, who was travelling with Ms Yamamoto, said it appeared she was shot by government forces.
Three other journalists - a Lebanese woman, an Arab male working for a US media outlet and a Turkish national - have been reported missing while covering the Syria uprising.
A total of 23,000 people have now been killed since March last year, according to the Syrian Observatory for Human Rights, while the UN puts the death toll at around 17,000.