WASHINGTON — President Trump signed an executive order on Tuesday resuming the admission of refugees to the United States under tighter security screening. But administration officials said they will subject 11 unidentified countries to another 90-day review for potential threats.

The order lifted a suspension on new refugee admissions that Mr. Trump first imposed shortly after taking office in January. At the time, it was part of a broader effort to limit the flow of foreigners admitted to the United States on the grounds of security, an initiative that has generated one of the sharpest legal and political debates of his nine-month-old presidency.

The president’s decision to lift the suspension, however, will not end the debate. Administration officials were vague about the additional 90-day review of the 11 countries, refusing even to name them, citing law enforcement sensitivities. They said refugees from those countries could still be admitted on a case-by-case basis during the 90 days if their entry is deemed in the national interest, and they do not pose a threat to the security or welfare of the United States.

It was not clear whether the new screening procedures would significantly diminish the chances for many applicants. While refugees who were vetted and approved before Mr. Trump took office have been allowed into the country this year, no new applications have been processed or approved since June.