The U.S. envoy for Venezuela, Elliott Abrams, said the protesters who have camped out at the embassy for more than two weeks are breaking the law and will have to leave.
Abrams refused to discuss possible timing of when the activists could be arrested.
Metropolitan Police spokeswoman Alaina Gertz said the State Department and the Foreign Missions Branch of the Secret Service are handling the protest.
Maduro closed the embassy in January and recalled his country's diplomats after the U.S. recognized opposition leader Juan Guaido as interim president. The U.S. and about 50 other nations view Maduro's re-election last year as illegitimate.
Activists from the group Codepink accuse the U.S. of trying to orchestrate a coup in Venezuela and have camped out at the closed embassy since April 10.
Abrams said on Thursday following a speech at the Atlantic Council that the activists are "clearly breaking the law."
Medea Benjamin, co-founder of Codepink, said she is one of a dozen people who have been sleeping inside the embassy and have been holding evening events.
"We feel this is part of an orchestrated coup that can lead to a civil war and can lead to tremendous violence," Benjamin said. "We want to say stop."
Benjamin said a Venezuelan diplomat loyal to Maduro had given her the keys to the embassy. She refused to identify the diplomat.
Abrams said that regardless of who handed the keys to Benjamin, they are trespassers because "there is a Venezuelan ambassador to the US, and he has told them to leave."
Carlos Vecchio, designated by Guaido as Venezuela's ambassador, refused to comment on Thursday morning.
In New York, Venezuelan Foreign Minister Jorge Arreaza said on Thursday during a press conference at the United Nations that the Venezuelan Embassy in Washington is the property of the Venezuelan government and that these demonstrators are helping Venezuela keep its building.
"We have to protect that Venezuelan building," Arreaza said. "It would be crazy to take it over illegally right? Have we proposed to do that in the U.S. Embassy in Caracas? Never."
Arreaza added that if the U.S. ejects the protesters, "We would have to think how to reciprocate in that case. I hope it does not happen."
Arreaza also said that he hopes the U.S. decides to respect Venezuelan property so the demonstrators soon will be able to go home.
The United Nations recognizes Maduro as Venezuela's president. The only two multilateral organizations that have recognized Guaido as president are the Organization of American States and the Inter-American Development Bank.
Associated Press writers Ashraf Khalil and Colleen Long in Washington, and Claudia Torrens in New York contributed to this report.
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