HAVANA — Cuba’s allies urged America to refrain from interfering with the communist country during Fidel Castro’s absence from power, while America increased its television transmissions to the island and encouraged anti-Castro activists to push for change.

The Cuban parliament speaker, Ricardo Alarcon, warned that America would face “hell” if it meddled with the Caribbean island.

“We demand that the government of the United States respect Cuba’s sovereignty,” a letter from 400 leftist intellectuals and human rights activists published yesterday in Cuba’s state-run newspapers said. “We must prevent a new aggression at all costs.”

American officials have repeatedly said they will not invade Cuba and that they wish only to see democracy on the island.

“Our desire is for the Cuban people to choose their own form of government,” President Bush said from his ranch in Crawford, Texas.

Yet many of those who fear an American attack on Cuba point to Iraq and Afghanistan — and the failed American-backed Bay of Pigs invasion of Cuba in 1961.

Any invasion now would “become a hell for them from the first day,” Mr. Alarcon said Monday.

“We will guarantee them total failure once again,” he said in an interview from Havana with the Venezuela-based television station Telesur, in an apparent reference to the Bay of Pigs attack.

Mr. Castro, who turns 80 on Sunday, is said to be recovering from intestinal bleeding that forced him to temporarily cede power a week ago to his younger brother, the defense minister, Raul Castro.

Neither of the brothers has made public appearances since.

The Communist Youth newspaper yesterday published a series of letters to Mr. Castro from children and teenagers across the country.

“We care about you so much, and since the moment of this sad news, haven’t stopped thinking about you,” Rina Forment, a 10-year-old in the eastern city of Santiago, wrote.

No details on Mr. Castro’s specific condition or what surgical procedure he underwent have been provided, with officials simply saying the Cuban leader’s health is rapidly improving and that he’ll be back to work soon.

Mr. Castro “continues to be coming along favorably, and we are sure that he will recover,” Vice President Carlos Lage, who was in Bogota for the inauguration of the Colombian president, Alvaro Uribe, said.

“He himself has said that, in a few weeks, he will be back at work again,”

Mr. Lage said, adding that Cuba was operating normally in the leader’s absence.

Mr. Bush said America was in the dark about Mr. Castro’s true health condition.

“The only thing I know is what has been speculated, and this is that, on the one hand, he is very ill, and, on the other hand, he is going to be coming out of hospital,” Mr. Bush said.

America planned to increase the television transmissions of its Miami-based TV Marti station to Cuba to six afternoons a week from one.

Congress approved $10 million in its 2006 budget to develop airborne TV broadcasting to counter the Cuban government’s mostly successful efforts to jam the transmission. A new private plane to be used for the transmissions was unveiled on Saturday.