UPDATE 1-Obama warns Syria's Assad against use of chemical weapons

WASHINGTON, Dec 3 (Reuters) - President Barack Obama issued

a warning to Syrian President Bashar al-Assad on Monday not to

use chemical weapons against Syrian opposition forces, saying

there would be consequences if he were to do so.

"I want to make it absolutely clear to Assad and those under

his command: The world is watching," Obama said in a speech to a

gathering of biological, chemical and nuclear weapons

proliferation experts.

"The use of chemical weapons is and would be totally

unacceptable and if you make the tragic mistake of using these

weapons, there will be consequences and you will be held

accountable," Obama said.

He did not say how the United States might respond, but

White House spokesman Jay Carney said earlier that "contingency

planning" was under way when asked whether the use of military

force was an option.

It was unclear what has motivated U.S. officials to think

that Syria might be on the verge of using chemical weapons.

An American official said the United States is concerned

that Syria may be preparing to combine the chemicals needed to

make sarin gas.

As Assad's government has shown signs of increasing strain

in response to recent advances made by the rebels, Carney said

the United States has grown concerned that the Syrian president

might be considering the use of chemical weapons.

This would, Carney said, "cross a red line for the United

States."

Some U.S. Republicans have been critical of the Obama

administration's response to the Syrian crisis as thousands of

people have been killed during the country's civil war.

During the presidential campaign, Obama's Republican

challenger, Mitt Romney, said the United States should

facilitate the arming of Syrian rebels, a step Obama has not

taken.

Obama said in his speech on Monday that the United States

would continue to support the legitimate aspirations of the

Syrian people, engaging with the opposition and providing them

with humanitarian aid. He said his goal is a transition in Syria

to a country that is free of Assad.