Several US governors have said they are putting on hold programmes to resettle Syrian refugees in their states.
Governor Rick Snyder of Michigan said he was suspending the acceptance of new refugees until a security review could be completed. And leaders in Alabama, Texas, Arkansas and Louisiana have said they would oppose the settlement of any more.
The decisions come in the wake of the attacks in Paris which killed 129 people on Friday evening.
Seven of the perpetrators died in the attacks, and one of them is thought to have been a Syrian who entered Europe via Greece with migrants.
Millions of Syrians have fled to neighbouring countries and to Europe, and the US has promised to take about 10,000 Syrian refugees in the next 12 months.