UN Secretary General Ban Ki-moon on Wednesday urged Syria's regime against using its stockpile of chemical weapons, warning of "huge consequences" if Bashar Assad resorts to such weapons of mass destruction.

Ban also suggested that he would not favour an asylum deal for the Syrian leader as a way to end the country's civil war and cautioned that the United Nations doesn't allow anyone "impunity."

"I again urge in the strongest possible terms that they must not consider using this kind of deadly weapons of mass destruction," Ban told The Associated Press, speaking on the sidelines of a climate conference in Qatar.

"I have warned that if in any case this should be used, then there will be huge consequences. And they should be accountable," he said of the Syrian regime.

Ban's comments came as NBC News, quoting unidentified U.S. officials, reported the Syrian regime has begun to load the precursor chemicals to make the nerve agent Sarin into bombs that could be dropped from Syrian fighter-bombers.

Chemical stockpile

Syria is believed to have hundreds, if not thousands, of tons of chemical agents, including mustard gas, a blistering agent, and the more lethal nerve agents Sarin and VX, experts say.

Damascus has said it would not use such weapons on his own people even if it had them. Syria is party to the 1925 Geneva Protocol banning chemical weapons in war.

U.S. intelligence has seen signs that Syria is moving materials inside chemical weapons facilities recently, though it is unsure what the movement means. Still, U.S. officials said the White House and its allies are weighing military options should they decide to secure Syria's chemical and biological weapons.

CNN reported Wednesday that U.S. military and intelligence officials are consulting with Syria's neighbours — Turkey, Israel and Jordan — about what to do if it looks as if Assad is about to launch a chemical attack on his own people.

In Qatar, the UN chief was asked about the potential for an asylum deal that would remove Assad from power. The Syrian president vowed in an interview with Russia Today last month that he would never be forced into exile and that he would "live and die in Syria."

"Whoever commits [a] gross violation of human rights must be held accountable and should be brought to justice. This is a fundamental principle," Ban said.

Fighting intensifies

Ban's warnings came as fighting around the Syrian capital, Damascus, was closing in on Assad's seat of power.

Clashes between rebels and regime troops have intensified in the suburbs ringing the city in recent weeks. The area has been a stronghold of predominantly Sunni Muslim rebels who are fighting to topple Assad's regime, dominated by Alawites, an offshoot Shia group.

The increased pressure of the opposition fighters on the capital has raised worries that Assad or his forces will resort to desperate measures, perhaps striking neighbours Turkey or Israel, or using chemical weapons.

Syria's uprising began with peaceful protests in March 2011 and later escalated into a civil war that the opposition says has killed more than 40,000 people. So far, both sides have refused international calls for a negotiated solution.