WASHINGTON — After pulling all remaining American diplomats from Venezuela on Thursday, the Trump administration warned the country’s leader, Nicolás Maduro, that any pressure on Juan Guaidó, the self-declared interim president who has the White House’s support, would be met with an “immediate reaction” from the international community.

But it remained unclear what that would mean. The administration issued the threat in the midst of a diplomatic retreat that has cast doubt on the United States’ ability to reinforce Mr. Guaidó if Mr. Maduro’s government moved to arrest him on accusations of orchestrating a dayslong blackout that plunged Venezuela deeper into unrest before power was restored in most major cities on Tuesday.

“We hold former President Maduro and those surrounding him fully responsible for the safety and welfare of interim president Juan Guaidó and his family,” Robert Palladino, a State Department spokesman, said on Thursday. “It would be a terrible mistake for the illegitimate Maduro regime to arrest Juan Guaidó.”

Mr. Palladino said the United States would be seeking to recruit a country to act as a protecting power and support any United States citizens still in Venezuela. But in a briefing with reporters, Mr. Palladino urged Americans to “strongly consider” departing the country.