Barack Obama has warned Syria's President Bashar al-Assad that the movement of chemical weapons would be a "red line" not to be crossed.
The Obama administration has thus far rejected calls to arm Syria's rebels, or send its own military force into the country. However, he now says the movement of chemical or biological weapons could change his view on the matter.
"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. That would change my calculus," Obama said during an unscheduled White House news conference.
"It doesn't just include Syria. It would concern allies in the region, including Israel, and it would concern us."
When asked about the use of US forces to safeguard the chemical weapons arsenal, he said: "We're monitoring that situation very carefully. We have put together a range of contingency plans."
Once again Obama called for Assad to step down and pave the way for a political transition. However, he added that "at this point, the likelihood of a soft landing seems pretty distant."
It has been feared for a long time that Assad may unleash chemical weapons in a bid to crush the rebels. Syria is thought to possess the largest stockpile of chemical weapons in the Middle East, and the fourth largest in the world.
It was recently reported that the large stockpile of chemical weapons had been moved to strategic locations in Syria.
Critics of Assad fear that the stockpile may fall into the hands of militants soon after the current regime is toppled. Israel had warned that if the Syrian-backed Hezbollah tried to capitalise on the current situation to capture the chemical weapons stockpile, it would respond with utmost force.
A Syrian foreign ministry spokesperson recently acknowledged the existence of such deadly weapons but vowed that the regime would not deploy them inside the country.