By Patricia Zengerle and Doina Chiacu
WASHINGTON (Reuters) - Republican lawmakers defied President Barack Obama on Wednesday and set out plans following last week's Paris attacks to tighten screening of Syrian refugees in a fight that challenges America's view of itself as a refuge for downtrodden immigrants.
Concerned about possible attacks in the United States after Islamic State killed 129 people in the French capital on Friday, the Republican chairman of a U.S. House of Representatives security panel proposed additional scrutiny of refugees fleeing Syria as well as Iraq seeking to enter the United States.
Reports that at least one of the Paris attackers was believed to have slipped into Europe among migrants registered in Greece prompted several Western countries to begin to question their willingness to take in refugees.
Under Republican-backed legislation introduced in the House, no refugee from war-torn countries Iraq and Syria could enter the United States until top-level American officials assure Congress that they do not imperil national security.
"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," said Michael McCaul, the Republican House Homeland Security Committee chairman.
The House is due to start debate on the bill on Thursday.
Obama has urged Congress not to "descend into fear and panic" and make it more difficult for refugees to enter the United States because of the Paris attacks.
"Slamming the door in the face of refugees would betray our deepest values. That's not who we are. And it's not what we're going to do," Obama said on Twitter from an Asia-Pacific summit in Manila.
Obama stood by a plan that the White House announced in September to allow 10,000 Syrian refugees into the United States within a year.
Refugees from the four-year-old civil war in Syria who are seeking U.S. entry already undergo a rigorous screening process that can take between 18 and 24 months, involving multiple U.S. security agencies.
House Speaker Paul Ryan said Republican efforts to heighten the screening of refugees will not discriminate against Muslims, who are in the vast majority in Syria and Iraq.
"We will not have a religious test, only a security test," Ryan said in a speech on the House floor. Some Republicans have said only Syrian Christians should be eligible for asylum in the United States.
The Senate, where Republicans hold a smaller majority than in the House, would have to approve any legislation on refugees before it could take effect. Rhetoric there about the issue has been less heated than in the House.
(Additional reporting by Susan Cornwell, Richard Cowan and Megan Cassella; Writing by Alistair Bell; Editing by Will Dunham)