Calls for a tougher U.S. policy against Syria intensified Thursday as a bipartisan group of senators urged President Barack Obama to threaten regime-toppling military action if President Bashar al-Assad uses chemical weapons against rebels trying to drive him from power.

Speaking at a hastily arranged news conference, Sens. John McCain, R-Ariz.; Lindsey Graham, R-S.C.; Joseph I. Lieberman, I-Conn.; and Chris Coons, D-Del., cited recent media reports about intelligence suggesting Syrian troops had mixed precursor chemicals for a deadly nerve gas and that the weapons might be loaded into bombs and artillery shells for use in the fighting.

The lawmakers also called on Obama to launch pre-emptive military action if the administration received “hard intelligence” of an Assad decision to actually use the chemical weapons.

“The time for talking about what to do may now be coming to a close,” said McCain, the ranking Republican on the Armed Services Committee, who has led the effort on Capitol Hill to arm the Syrian rebels since their insurgency began nearly two years ago.

He said the administration now has to decide “whether to continue on the sidelines and hope that a man who has slaughtered nearly 40,000 men, women and children in Syria will decide not to take the next step and use far more destructive weapons to kill significantly larger numbers of people, or whether to take military action of some kind that could prevent a mass atrocity. That is the choice we now face.”

Graham said he was preparing a resolution authorizing the use of force to prevent Assad from using weapons of mass destruction against the insurgents.

“If he does, it would be the end of his regime,” Lieberman said.

Obama, Secretary of State Hillary Rodham Clinton and NATO leaders have warned that Syria’s use of chemical weapons would be met with a strong international response, but none has specified what the United States and its allies would do in the event such weapons were used.

“I don’t want to pre-suppose what the president is going to do with the Cabinet, but I think what must be understood is that the use of those weapons is for us a qualitatively different situation and frankly countries in the region also view it that way,” Robert S. Ford, the U.S. ambassador to Syria, said Thursday at a forum hosted by the Foundation for Defense of Democracies think tank. “And so it will change our calculations in a fundamental way, change completely the approach to the Syrian problem.”

Ford added, “We want to be very clear, very clear to the Syrian government, as its situation deteriorates, they must not deploy these weapons, they must not deploy them.”

Until now, the United States has refused to arm the rebels, fearing such weapons might fall into the hands of Islamic extremists fighting the Syrian regime. Instead, Washington has provided the rebels only with humanitarian aid and communications gear.

The reports of Syrian troops preparing chemical weapons appeared as representatives of 70 nations calling themselves the “Friends of Syria” prepared to meet Dec. 12 in Marrakesh, Morocco, to discuss the crisis. At the gathering, Clinton was expected to recognize the recently formed Syrian opposition, a broad coalition of rebel groups both inside and outside Syria.

The New York Times on Wednesday reported that U.S. intelligence officials had detected that Syrian troops have mixed together small amounts of precursor chemicals for sarin, a deadly nerve gas, at one or two storage sites, and that the chemical weapons could be used in bombs and artillery shells. Syria has said it would not use such weapons against its own people “in any circumstances.”

The four senators also urged the administration to consider the use of military force to secure Syria’s chemical weapons if Assad’s regime collapses.

“There is a unanimous view here today that we support President Obama in announcing a red line and prompt consequences should Bashar al-Assad take the unthinkable step of using advanced chemical weapons against his own people, said Coons, a member of the Senate Foreign Relations Committee and the first Democrat to join the group of Syria hawks.

Separately, Sen. Bob Casey, D-Pa., chairman of the Foreign Relations Subcommittee on Near Eastern and South and Central Asian Affairs, on Thursday laid out other steps he thinks the Obama administration should take to assist the Syrian opposition that go further than anything he has advocated to date.

Speaking at the Foundation for Defense of Democracies event, Casey called on the administration to consider taking out Syrian government fighters by using surface-to-surface Tomahawk missiles. He emphasized the targets would be Air Force planes about to take off, rather than in the air.

He added that the administration should consider other ways the United States can “hamper the ability of the Syrian Air Force to conduct aerial attacks on civilians.”

Casey also said the administration should consider initiating monetary support and security cooperation with “heavily vetted opposition groups that are committed to the democratic process and universally accepted human rights.” Security cooperation would include training and intelligence sharing, but not, notably, providing arms, as some other lawmakers have advocated.

“If we can help them, and help them on this front in a way that does not exacerbate international opposition dynamics, then we should,” he said.

Lawmakers’ push for new U.S. action comes as the momentum on the ground in Syria appears to be shifting in the rebels’ favor.

“I think the direction of events on the ground, the direction is very clear now,” Ford said. “When Damascus airport, which is not far from the city, they have to close it because it comes under attack of armed opposition forces … when armed opposition groups seize control of air defense sites inside the outer Damascus beltway, the writing is on the wall.”