A U.S. lawmaker introduced a bill aiming to toughen the vetting process for refugees seeking to enter the United States as Republican leaders in Congress sought to block Syrians fleeing war in their country.

The Republican chairman of the House of Representatives Homeland Security Committee, Michael McCaul, said his bill would strengthen security measures for all refugee populations.

It would stipulate that no Syrian or Iraqi refugee can enter the United States until Congress receives certification that they are not a national security threat, he said in a statement late on Tuesday.

"The bill requires the nation's top security officials - the Secretary of Homeland Security, the Director of the FBI, and the Director of National Intelligence - to certify before admitting any Syrian or Iraqi refugee into the United States that the individual does not represent a security threat," McCaul said.

House Republican leaders, worried about Islamic State attacks after Friday's killings of 129 people in France, on Tuesday threatened to suspend the administration's plans to admit 10,000 Syrian refugees in the coming year.

McCaul said legislation was necessary because the president was unlikely to halt the program.

Obama administration officials have said Syrians seeking to enter the United States undergo the toughest security screening of any group. Obama, traveling in Asia, on Tuesday called attempts to block entry "offensive and contrary to American values."

On Wednesday, White House spokesman Josh Earnest dismissed legislation urging a halt to the refugee program.

"The fact of the matter is legislation like that is much more focused on politics than it is on national security," Earnest said in an interview with MSNBC television channel.