WASHINGTON - US President Barack Obama on Monday warned the Syrian government against using chemical weapons in conflicts with the opposition forces or letting them fall into the wrong hands, calling it a "red line" that would change his calculus in his approach to the prolonged conflict in the Middle East nation.

The issue of chemical and biological weapons concerns not only Syria, but also Washington's close allies in the region, including Israel, and the United States itself, Obama said.

"We cannot have a situation where chemical or biological weapons are falling into the hands of the wrong people," he told an impromptu White House news conference.

"We have been very clear to the al-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 utilized," he said, stressing "That would change my calculus. That would change my equation."

Syria's stockpile of chemical weapons, which Western powers believe is one of the world's largest, has been a growing concern for Washington and its allies as unrest in the Arab nation has dragged on for 18 months.

The Syrian government has said that it will never use its stockpile, believed to include sarin nerve agent, mustard gas and cyanide, in the ongoing conflicts with the opposition forces, except in case of "external aggression."

Obama called the Syrian conflict "a very tough issue," saying he has not ordered military engagement in the situation there, but rather focused on providing humanitarian assistance to the Syrians affected as well as aid to the opposition in managing a political transition.

He reiterated Washington's long-held call for Syrian President Bashar al-Assad to step down, saying "The international community has sent a clear message that rather than drag his country into civil war, he should move in the direction of a political transition."

"But at this point the likelihood of a soft landing seems pretty distant," Obama remarked.

He said Washington is monitoring the situation very carefully and has put together "a range of contingency plans."