In Washington, the U.S. State Department said procedures under the Venezuelan Constitution should be followed if Chavez is no longer able to carry out his duties as president.
"We want to see any transition take place in a manner that is consistent with the Venezuelan Constitution, that any election be fully transparent, democratic, free and fair," State Department spokeswoman Victoria Nuland told reporters on Wednesday.
Asked if Chavez being out of the picture would make it easier to improve long-strained ties between Venezuela and the U.S., Nuland said, "Obviously we will judge our ability to improve our relationship with Venezuela based on steps they are able to take."
The U.S. Embassy in Caracas has been without an ambassador since July 2010. Chavez rejected the U.S. nominee for ambassador, accusing him of making disrespectful remarks about Venezuela's government. That led Washington to revoke the visa of the Venezuelan ambassador.
But recently U.S. and Venezuelan diplomats began high-level conversations aimed at improving relations, a U.S. government official said. The official spoke on condition of anonymity because he wasn't authorized to speak publicly about the matter.
The official confirmed recent reports that Roberta Jacobson, assistant secretary of state for Western Hemisphere affairs, spoke by telephone with Maduro in November and discussed ways of improving relations. He also confirmed that U.S. diplomat Kevin Whitaker had a subsequent conversation with Roy Chaderton, Venezuela's ambassador to the Organization of American States.
Venezuelan diplomats could not be reached to comment about those recent contacts with U.S. officials.
In Bolivia, meanwhile, President Evo Morales said he is concerned about his friend and ally.
"I hope we can see him soon," Morales said at a news conference Wednesday. "But it's a very worrying situation."
"I've tried to make contact with the vice president, and it's been difficult. I hope all of their aims are achieved to save President Chavez's life."
Before his operation, Chavez acknowledged he faced risks and designated Maduro as his successor, telling supporters they should vote for the vice president if a new presidential election were necessary.
Maduro didn't discuss the upcoming inauguration plans, saying only that he is hopeful Chavez will improve.
"Someone asked me yesterday by text message: How is the president? And I said, 'With giant strength,'" Maduro said. He recalled taking Chavez by the hand: "He squeezed me with gigantic strength as we talked."
Associated Press writers Jorge Rueda in Caracas, Luis Alonso Lugo in Washington and Paola Flores in La Paz, Bolivia, contributed to this report.
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