WASHINGTON, D.C.: President Barack Obama dramatically told Syria’s President Bashar al-Assad Monday (Tuesday in Manila) not to turn chemical weapons on his own people, following US warnings his forces were mixing deadly sarin gas.

Obama publicly told the increasingly isolated Assad not to unleash the “worst weapons of the 20th century” in the 21st, capping a day of alarming American warnings on the Syrian regime’s intentions.

“Today, I want to make it absolutely clear to Assad and those under his command, the world is watching, the use of chemical weapons is and would be totally unacceptable,” Obama said. “If you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable.”

The Damascus government, hitting back at increasingly explicit US rhetoric, had earlier pledged never to take such a step, which the Obama administration warns would cross a “red line” and result in US action.

Adding to the impression of a quickening endgame in Syria, Arab League chief Nabil al-Arabi exclusively told Agence France-Presse that Assad’s regime was in danger of collapse “anytime” as the opposition makes sweeping gains.

And prompted by deteriorating security, the United Nations suspended operations in Syria and said it would pull out non-essential staff, while the European Union reduced its activities in Damascus to a minimum.

A US official told Agence France-Presse that Syria had begun mixing chemicals that could be used to make sarin, a deadly nerve agent, while CNN reported Damascus could deploy the gas in a limited artillery attack on advancing rebels.

The White House has been loath to make a direct intervention in Syria but indicated Monday that the use of chemical weapons could change the equation.

“We are concerned that an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people,” White House spokesman Jay Carney said.

Washington worries that battlefield advances by rebels could prompt Assad to use chemical arms, or that such stocks could become insecure or find their way into the hands of groups hostile to the United States and allies.

In televised remarks, an unnamed Syrian foreign ministry official said Syria would “never, under any circumstances, use chemical weapons against its own people, if such weapons exist.”

Jordanian Foreign Minister Nasser Judeh said in Washington that the use of chemical weapons in Syria would be a “game changer.”

“The regime knows that the international community will simply not accept for these weapons to be used in different scenarios: whether they are used by the regime against their own people or used against neighboring countries or fall into the wrong hands,” he said.

In Syria itself, there were new developments in a vicious conflict that has taken an estimated 41,000 lives since erupting in March 2011.

Arabi said Assad’s regime was in danger of collapse “anytime” as the opposition made political and military headway.

“Facts on the ground indicate very clearly now that the Syrian opposition is gaining, politically and militarily. Every day they are gaining something,” Arabi said.

In another blow to the Assad regime, foreign ministry spokesman Jihad Makdisi, a prominent advocate of the president, was reported to have quit and headed for London from Beirut.