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".
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".