WASHINGTON (Reuters) - The United States has recently rejected 30 Syrians out of thousands seeking to enter the country, Obama administration officials told a congressional panel on Wednesday, as the United States tightens vetting of immigrants and other visitors following attacks in California and Paris.

In addition, hundreds of applications from Syrian refugees have been put on hold and many might ultimately be rejected, Leon Rodriguez, director of the Citizenship and Immigration Services unit of the Department of Homeland Security, told the committee.

A spokesman for Rodriguez later said that the 30 Syrian refugee applications had been rejected over the last 16 months.

At a time when millions of refugees are arriving in Europe and elsewhere from the Middle East and Africa, Democratic President Barack Obama’s pledge to take in 10,000 people fleeing war-torn Syria has come under fire, especially from Republicans. The United States so far has admitted 2,000 refugees.

Michael McCaul, the Republican chairman of the House of Representatives Homeland Security Committee, said at a hearing that authorities have identified about 40 violent militants who had attempted to enter Europe posing as refugees.

Other committee Republicans at the hearing questioned why the Obama administration wanted to admit any Syrian refugees, given that the Syria-based Islamic State movement has pledged to attack the United States and other western countries.

“Our intelligence community has ... told me that individuals with terrorism ties in Syria have already tried to gain access to our country through the refugee program,” McCaul said.

“What’s even more concerning is that top officials have testified before this committee that intelligence gaps prevent us from being able to confidently weed out terrorists,” he said.

Rodriguez and other Homeland Security and State Department officials told the committee that U.S. procedures for vetting Syrian refugees were among the most rigorous in the world.

U.S. agencies have tightened procedures for checking backgrounds of would-be U.S. immigrants and visitors after a recent arrival from the Middle East was one of two shooters who killed 14 people in San Bernardino, California.

Francis Taylor, the Homeland Security Department’s intelligence chief, said his department was routinely doing social media checks on would-be immigrants and visitors.