US State Department subjects Syrian refugees to the most intensive security checks of immigrants as concerns raised after the deadly Paris attacks.

"These refugees are subject to the highest level security checks of any category of traveler to the United States," State Department spokesman Mark Toner told a daily news briefing. "We think we can do this safely and in a way that reflects American values."

Also on Monday, several US governors have threatened to stop accepting Syrian refugees as the weekend attacks in Paris raised security worries over Syrian migrants.

Concerns highlighted after the attacks that terrorists might use the refugees as cover to sneak across borders.

One of the terrorists involved in Paris attacks was found to have a Syrian passport, and likely made it to the country in the cover of refugees, according to French police.

As millions of Syrians have fled to neighboring Middle Eastern countries and Europe, US President Barack Obama had announced that his administration would admit at least 10,000 Syrian refugees in the coming year.