US President Barack Obama has declared the threat of chemical or biological warfare in Syria a "red line" for the United States.
With that statement, he is outlining for the first time the point at which his administration could feel forced to intervene militarily in the Arab country's increasingly messy conflict.
Speaking to reporters Monday at the White House, Obama warned about the use or deployment of such weapons of mass destruction.
He says they risk widening a civil war that already has dragged on for one-and-a-half years and, activists say, killed some 20,000 people.
It's widely thought that Syria possesses extensive chemical and biological weapon stockpiles. Syria has threatened to use them if the country comes under foreign attack.
Russia warned the West on Tuesday against unilateral action on Syria, a day after Obama threatened "enormous consequences" if his Syrian counterpart used chemical or biological arms or even moved them in a menacing way.
Russian foreign minister Sergei Lavrov, speaking after meeting China's top diplomat, said Moscow and Beijing were committed to "the need to strictly adhere to the norms of international law...and not to allow their violation".
Russia and China have opposed military intervention in Syria throughout a 17-month-old revolt against President Bashar al-Assad.
They have vetoed three UN Security Council resolutions backed by Western and Arab states that would have put more pressure on Damascus to end violence that has cost 18,000 lives.
The US and its allies have shown little appetite for military action in Syria, in contrast to last year's Nato campaign that helped topple Libyan leader Muammar Gaddafi.
But Obama said on Monday he had refrained "at this point" from ordering US military engagement in Syria.
But when asked at a White House news conference whether he might deploy forces to secure chemical weapons, he said his view could change.