San Miguel is president of the non-governmental organization Control Ciudadano, which advocates for citizen oversight of Venezuela's armed forces. She was arrested this week on charges of involvement in an alleged plot to assassinate President Nicolas Maduro.
"We are watching this very, very closely," White House spokesperson John Kirby told reporters, urging Maduro to meet commitments his government made in a deal with the political opposition to hold elections this year.
"Mr. Maduro needs to meet the commitments that he made back in the fall about how they are going to treat civil society, political activists, as well as opposition parties," Kirby said.
Venezuela's attorney general, Tarek Saab, said on Sunday that San Miguel had been arrested for "allegedly being linked and referenced in the conspiracy plot and attempted assassination ... aimed at attacking the life of Head of State Nicolas Maduro and other high-ranking officials."
San Miguel's legal team says it has requested information from Venezuelan authorities following her arrest but has not received a response. Human rights groups say her lawyers were not present at her arraignment on Monday.
Calling for her immediate release and right to legal defense, the United Nations' human rights office said San Miguel's "whereabouts remain unknown, potentially qualifying her detention as an enforced disappearance."
Maduro's election deal with the opposition caused the U.S. to temporarily ease economically debilitating oil sanctions on crude-exporting Venezuela. Washington began reimposing sanctions last month after Venezuela's top court upheld a ban blocking the candidacy of the leading opposition presidential hopeful.
(Reporting by Jeff Mason with additional reporting by Gabriel Araujo and Mayela Armas; editing by Rami Ayyub)
