Updated 4:53 PM ET
Venezuela's vice president, Nicolas Maduro, says the government of President Hugo Chavez plans to expel a U.S. Embassy official for meeting with military officers and planning to destabilize the country.
Maduro identified the American as the Air Force attache and said he had been spying on the military.
He said the official has 24 hours to leave the country.
In response to the accusations, a State Department official told CBS News: "We are aware of the allegations made by Venezuelan Vice President Maduro over state-run television in Caracas, and can confirm that our Air Attache, Col. David Delmonico, is en route back to the United States."
Foreign Minister Elias Jaua also announced the expulsion of a second U.S. official, also a U.S. Air Force attache. A Pentagon told CBS News: "We completely reject the Venezuelan government's claim that the United States is involved in any type of conspiracy to destabilize Venezuela government. Further, we absolutely reject the specific allegations leveled by the Venezuelan government against Air Attache David Delmonaco and Assistant Air Attache Devlin Kostal."
Maduro spoke hours after the government said Chavez was in "very delicate" health after undergoing cancer surgery in December.
Maduro said on state television that a scientific commission will investigate the possibility Chavez's illness was caused by an enemy attack.
The government announced Monday night that Chavez was in "very delicate" condition after suffering a new, severe respiratory infection.
Chavez anointed Maduro as his preferred successor before flying to Cuba in early December to undergo a fourth round of surgery for an unspecified cancer in his pelvic area.
The constitution calls for snap presidential elections if Chavez is declared incapable of governing, or dies.
Chavez has run Venezuela for more than 14 years as a virtual one-man show, gradually placing all state institutions under his personal control. But the former army paratroop commander, who rose to fame by launching a failed 1992 coup, never groomed a successor with his same kind of force of personality.
Chavez was last re-elected on Oct. 7, and his challenger, youthful Miranda state Gov. Henrique Capriles, is expected to again be the opposition's candidate in any new election.
One of Chavez's three daughters, Maria Gabriela, expressed thanks to well-wishers via her Twitter account. "We will prevail!" she wrote, echoing a favorite phrase of her father. "With God always."
Maduro said last week that the president had begun receiving chemotherapy around the end of January.
Doctors have said such therapy is not necessarily to beat Chavez's cancer into remission, but could have been palliative, to extend Chavez's life and ease his suffering.
Dr. Carlos Castro, scientific director of the Colombian League Against Cancer, said "it's difficult to predict" when Chavez might die, but he believes "it's a matter of days."
Castro said Chavez could face further respiratory complications if he receives more intense chemotherapy treatment.
If the president's medical team "gives him strong chemotherapy again, then it would not be surprising if some infections reappear," Castro said in a telephone interview.
While in Cuba, Chavez suffered a severe respiratory infection in late December that nearly killed him, Maduro said last week. A tracheal tube was inserted then, and government officials have said his breathing remained labored.
Libardo Rodriguez, a 60-year-old orange juice vendor, said he was very worried after Monday evening's announcement and the government should provide more information about Chavez.
"We are worried because he does not appear. The truth is that I don't know what's happening," said Rodriguez, who identified himself as a Chavez supporter.
"There are many rumors and nobody knows who to believe," he said. "We hope he's alive."