He signed the executive order at the Pentagon after a ceremony to swear in General James Mattis as defence secretary.
He did not elaborate on the vetting measures.
But in a T-V interview broadcast on Friday, he said Christians would be given priority among Syrians who apply for refugee status.
He has also vowed to curb entry for people from six other Muslim countries.
He says the measures are necessary to stop terrorists entering the United States disguised as refugees.
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