Lawmakers in the United States moved Wednesday to freeze White House plans to resettle Syrian refugees, intensifying a standoff with President Barack Obama who had earlier accused Republicans of “hysteria” following the Paris attacks.

Republican leaders introduced legislation requiring assurances of more robust background checks and vetting before the White House can go ahead with its plan to welcome 10,000 refugees from the conflict in the coming year.

A vote on the measure, unveiled by House Homeland Security chairman Michael McCaul, could come Thursday.

“It does put the brakes on the program until they can properly vet and certify that these individuals do not pose a threat to the national security of the United States,” McCaul said.

The bill is aimed at strengthening vetting procedures for Syrian as well as Iraqi refugees in the wake of Friday’s attacks that killed 129 people in Paris.

The discovery of a Syrian passport near the body of one assailant in the Paris attacks has stirred fears among American lawmakers and governors that jihadists are seeking to blend in with refugee masses in order to strike later.

In fact, no less than 24 states announced earlier this week they would block the program to resettle Syrian migrants within their borders, though White House officials defended the current refugee program.

“I don’t think we can afford to play Russian roulette with our national security,” McCaul said Wednesday.

Amid intensifying concern over extremists from the Islamic State group infiltrating the West, and as U.S. lawmakers digest reports that several Paris attackers were French nationals, McCaul said lawmakers were also drafting legislation that would tighten the existing visa waiver program.

The program allows citizens of certain countries, including France and others in Europe, to travel to the United States without a visa.

“Obviously that’s a vulnerability when you have 5,000 foreign fighters with Western passports. We need to tighten up those security gaps,” McCaul said.

In an unusually fierce rebuke on the refugee issue, Obama struck out at Republicans, accusing them of demonizing “widows and orphans.”

“We are not well served when in response to a terrorist attack we descend into fear and panic,” Obama said Wednesday from Manila.

“We don’t make good decisions if it’s based on hysteria or an exaggeration of risks.”

McCaul’s plan would require the director of the FBI, the Secretary of Homeland Security and the Director of National Intelligence to certify that each refugee is not a security threat.

“With that comes great liability,” declared McCaul.

Once the top officials sign off on a determination that a refugee is not a security threat, “they own it, it’s their responsibility,” McCaul added.

House Speaker Paul Ryan backs the bill, telling colleagues that it would pause the refugee program until it was certain “beyond any doubt” that the new Syrian and Iraqi settlers do not pose a threat.

“Our first priority is to protect the American people,” he said. “We can be compassionate, and we can also be safe.”

Senator John McCain, a foreign policy hawk, said he supported a pause but offered a word of caution.

“I believe the overwhelming focus on the refugee program in recent days is misplaced,” he said. “I especially encourage my fellow Republicans to recognize that refugees are not the problem — they are the symptom of the problem.”

AFP contributed to this report.