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Republicans in the United States have intensified their opposition to letting thousands of Syrian refugees come to the US in the wake of the deadly Paris attacks.

Republican presidential contender Marco Rubio on Sunday said the United States could not admit Syrian refugees because it was impossible to know whether people fleeing Syria had links to terror groups - an apparent shift from earlier statements in which he left open the prospect of refugees being admitted with proper vetting.

"It's not that we don't want to, it's that we can't," Rubio said Sunday on the ABC network's This Week programme.

Other Republican candidates have called for a ban on allowing Syrians into the US

"There's no way to background check someone that's coming from Syria," Rubio said. "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 Governor Jeb Bush on Sunday told NBC's Meet the Press that the US should admit Syrian Christians, after proper vetting.

Other Republican candidates have called for a ban on allowing Syrians into the US, while all three Democratic presidential candidates have said they would admit Syrians after thorough background checks.

But Friday night's mass killings in Paris, which left at least 129 people dead, have rekindled suspicions that people with secret ties to terror groups could flow across borders as part of waves of refugees.

French authorities said a Syrian passport found near one of the Paris attackers had been registered last month and used to travel through three countries along a busy migrant corridor known for lax controls.

It has been reported that the passport was a fake.

Officials on Sunday were still trying to identify people involved in the attack. 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 group, it would be "almost malpractice" not to do everything possible to infiltrate the refugee ranks with militants bent on waging jihad.

A spokesman for President Barack Obama that the administration was 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, Senator Bernie Sanders and former Maryland Governor Martin O'Malley - said the US 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.

Jeb Bush, meanwhile, said that the US had a responsibility to "help with refugees after proper screening".