The deadly Paris attacks have intensified Republican opposition to letting thousands of Syrian refugees come to the United States.

Republican presidential contender Marco Rubio on Sunday said the United States cannot do so because it's impossible to know whether people fleeing Syria have links to Islamic militants — an apparent shift from earlier statements in which he left open the prospect of migrants being admitted with proper vetting.

"It's not that we don't want to, it's that we can't," Rubio said Sunday on ABC's This Week. "Because there's no way to background check someone that's coming from Syria. Who do you call and do a background check on them?"

The question of admitting Syrian refugees has for months been part of the national security discussion among 2016 candidates that cuts to the heart of the American identity as a refuge.

Former Florida Gov. Jeb Bush on Sunday told NBC's Meet the Press that the U.S. should admit Syrian Christians, after proper vetting. Other Republican candidates have called for a ban on allowing Syrians into the U.S. All three Democratic presidential candidates have said they would admit Syrians but only after thorough background checks.

But Friday night's mass killings in Paris, which left at least 129 people dead, offered evidence that may have backed up what many, including Rubio, had been warning: People with secret ties to Islamic militants could flow across borders as part of waves of refugees.

Jeb Bush

Republican presidential candidate and former Florida Gov. Jeb Bush said he would admit Syrian refugees that were Christian, after proper vetting. (Jim Cole/The Associated Press)

Authorities said a Syrian passport found near one of the Paris attackers that had been registered last month and travelled through three countries along a busy migrant corridor known for lax controls. It was not clear whether the document was real or forged. Officials on Sunday were still trying to identify people involved in the conspiracy. They said as many as three of the seven suicide bombers who died in the attacks were French citizens.

Republican presidential contender Ben Carson, a retired brain surgeon, said that from the viewpoint of the Islamic State of Iraq and Syria (ISIS) group, it would be "almost malpractice" not to do everything possible to infiltrate the refugee ranks with militants bent on waging jihad.

"As I've said all along, you know, bringing people into this country from that area of the world I think is a huge mistake. Because why wouldn't they infiltrate them with people who are ideologically opposed to us?  It would be foolish for them not to do that," Carson told Fox News Sunday. He said that the U.S. should use its resources to help refugees relocate closer to their country of origin.

Obama reaffirms prior commitment

A spokesman for President Barack Obama said Sunday that the administration is moving forward with its plan to thoroughly vet and admit as many as 10,000 Syrian refugees.

The Paris attacks have elevated national security in the presidential contest. In Saturday night's Democratic presidential debate, which began with a moment of silence for the Paris victims, all three candidates — former secretary of state Hillary Rodham Clinton, Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley — said the U.S. should admit far more than the 10,000 Syrians to which Obama has committed, but only with proper screening.

Rubio on Sunday said that was impossible.

"You can't pick up the phone and call Syria, and that's one of the reasons why I said we won't be able to take more refugees," Rubio said on ABC.

Former Florida Gov. Jeb Bush, meanwhile, said Sunday that the U.S. has a responsibility to "help with refugees after proper screening."

Meanwhile, Alabama Gov. Robert Bentley announced late Sunday he is refusing Syrian refugees relocating to his state.

In a news release Sunday Bentley said, "After full consideration of this weekend's attacks of terror on innocent citizens in Paris, I will oppose any attempt to relocate Syrian refugees to Alabama through the U.S. Refugee Admissions Program. As your Governor, I will not stand complicit to a policy that places the citizens of Alabama in harm's way."

One of the recognized State Department refugee processing centres in the U.S. is in Mobile, Ala.