The refugee policy under consideration would halt admissions from Syria and suspend it from other majority-Muslim nations until the administration can study how to properly vet them. This would pave the way for the administration to slash the number of displaced people who can be resettled on American soil, and would effectively bar the entry of people from Muslim countries — including Afghanistan, Iraq, Somalia and Syria — at least for some time.

The plan is in line with a ban on Muslim immigrants that Mr. Trump proposed during his campaign, arguing that such a step was warranted given concerns about terrorism. He later said he wanted to impose “extreme vetting” of refugees from Syria and other countries where terrorism was rampant, although the Obama administration had already instituted strict screening procedures for Syrian refugees that were designed to weed out anyone who posed a danger.

The expected actions drew strong criticism from immigrant advocates and human rights groups, which called them discriminatory moves that rejected the American tradition of welcoming immigrants of all backgrounds.

“To think that Trump’s first 100 days are going to be marked by this very shameful shutting of our doors to everybody who is seeking refuge in this country is very concerning,” said Marielena Hincapié, the executive director of the National Immigration Law Center. “Everything points to this being simply a backdoor Muslim ban.”

For Mr. Trump, whose raucous campaign rallies frequently featured chants of “build the wall,” the directive to fortify the border was not unexpected, although it may not be enough by itself to accomplish the task. Congress would need to approve any new funding necessary to build the wall, which Mr. Trump has insisted Mexico will finance, despite its leaders’ protestations to the contrary. The order would shift already appropriated federal funds to the wall’s construction, but it was unclear where the money would come from.