Barack Obama has warned that US forces could move against Syrian leader Bashar al-Assad if he deploys chemical weapons against rebels trying to overthrow him.
The US president's comments came as a female Japanese reporter was killed while covering clashes in the northern city of Aleppo.
In some of his strongest language yet on Syria, Obama told a White House news conference on Monday that Assad faced "enormous consequences" if he crossed a "red line" of even moving unconventional weapons in a threatening manner.
Obama noted that he had refrained "at this point" from ordering US military engagement, but when he was asked whether he might deploy forces, for example to secure Syrian chemical and biological weapons, he said his view could change.
"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilised," he said. "That would change my calculus."
Faced with a complex and explosive conflict, and with resolute support for Assad from Iran, and from Russia and China at the UN, Washington and its Western allies have shown little appetite for more than hands-off help for the rebels.
The stance is in contrast to their attacks on Libya's Muammar Gaddafi last year.
Last month, Syria acknowledged for the first time that it had chemical and biological weapons and said it could use them if foreign countries intervened.
The threat drew strong warnings from Washington and its allies, although it is not clear how the Syrian armed forces might use such weapons in urban warfare.
"We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," Obama told the impromptu news conference on Monday.
He acknowledged he was not "absolutely confident" the stockpile was secure.
Activists said more than 130 people were killed in Syria on Monday.
In-depth coverage of escalating violence across Syria
Six people, including two children and two women, were reported killed in the southern city of Daraa, birthplace of the country's 18-month-old uprising.
In the commercial capital Aleppo, nine civilians were said to have died, including two women, a nine-year-old boy and a five-year-old girl.
A female Japanese journalist was killed after being caught in gunfire in Aleppo, the foreign ministry in Tokyo said on Tuesday.
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, the company's website said.
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.
The opposition Syrian National Council said 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.
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.