President Obama has warned Syrian President Bashar al-Assad that he would be held accountable if he made the "tragic mistake" of using his stockpile of chemical weapons.
Obama addressed veterans in Reno, Nevada, with a speech on foreign policy.
He said: "Today we're also working so that the Syrian people can have a better future, free of the Assad regime".
"Given the regime's stockpiles of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching and that they will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," he said.
His sentiments have been echoed by France's Foreign Minister Laurent Fabius.
He said in a TV interview: "Any use of chemical arms is completely unacceptable. President Obama made statements to this effect and so have others... These weapons are under strict surveillance by the international community, "
The warnings come after the Syrian Foreign Ministry spokesman Jihad Makdissi acknowledged for the first time that his country had chemical and biological weapons.
He said on Monday the Syrian army would not use chemical weapons to crush rebels but could use them against forces from outside the country.
The announcement comes as over 1,200 people have been killed since Sunday in the bloodiest days of fighting in the uprising so far.
At least 300 Government troops are among those to have perished - alongside an unknown proportion of civilians and rebel fighters.