The head of the State Department's refugee bureau told a congressional panel on Thursday that Syrians trying to enter the United States pose "less of a threat" than other populations, sparking a harsh exchange with a Republican lawmaker.

Asked whether the U.S. government tracks Syrian refugees once they are in the country, Anne Richard, assistant secretary of state for population, refugees and migration, said authorities track them for three months after arrival.

"They're not treated differently than other refugees. Syrians are less of a threat, actually, because they've fled their country. They've voted with their feet," Richard said.

"Let me stop you there real quick," Republican Representative Lamar Smith of Texas interrupted.

He cited earlier testimony from FBI Director James Comey that he was worried about not having enough information on the refugees.

"Terrorist organizations have already said they're going to use the refugee program to try to infiltrate the United States and you say you're less worried about Syrian refugees than other refugees?" Smith asked incredulously.

Richard said the Syrian refugees have been out of Syria for a while and U.S. officials had a record of the time they have spent since they left. She said she was "very worried about terrorists" and that the refugee vetting process is focused on weeding out those threats.

"Don't you think Syrian refugees might someday become terrorists?" Smith persisted.

"I think the odds of a refugee being a terrorist is very, very small," Richard said. "But that doesn't stop us from focusing our program to make sure nobody comes in who might be a terrorist."

The testy exchange before the House Judiciary Committee came a day after congressional Republicans laid out plans to tighten screening of Syrian refugees after the Paris attacks, setting up a political battle with President Barack Obama.