(Adds quotes from Raul Castro, details, byline)

By Jeff Franks

HAVANA, Jan 21 (Reuters) - Cuban President Raul Castro dismissed rumors that his older brother Fidel Castro was at death's door, saying on Wednesday he was mentally and physically active despite a long illness that has kept him out of public view.

Speculation had been rising that the ailing 82-year-old was in serious condition after he failed to attend celebrations for the 50th anniversary of the Cuban revolution that put him in power on Jan. 1, 1959 and stopped publishing in mid-December what had been a steady stream of newspaper columns.

Fidel Castro also had not received recent visiting heads of state, but on Wednesday he met with Argentine President Cristina Fernandez, who said they talked for more than an hour, mostly about new U.S. President Barack Obama.

"He spent time with the president, he is exercising, thinking a lot, reading a lot, assisting me and helping," Raul Castro said. "Soon I'm going to make a trip to Europe. Do you think I could leave from here if Fidel was gravely ill?"

Speaking to reporters at the Havana airport as he was seeing off Fernandez after her three-day Cuba visit, Raul Castro said the speculation about his brother was all wrong and told the press to publish it "so the rumors end."

Fernandez also moved to quash the rumors about Fidel Castro's poor health.

"We talked about everything. He looked very good to me. We talked about the international situation," she was quoted as saying by Argentina's official news agency, Telam.

Fidel Castro has not been seen in public since July 2006 when he underwent surgery for an undisclosed intestinal ailment. He maintained a public profile through his writings, but they suddenly stopped after a Dec. 15 column.

Raul Castro provisionally took power after the surgery, then officially became president in February when his brother said he was not well enough to continue.

The Cuban president did not say why his brother's columns, published in state-run press, had ceased, but promised they would soon return.

Fernandez said Castro wore a blue jogging suit during their meeting and that he told her he had followed Obama's inauguration on Tuesday on television and had a good opinion of the United States' first black president.

"With much passion, with much conviction, he told me he's a sincere man, believes absolutely in everything he's saying, he has many good ideas and a very good history," Fernandez said.

Obama has said he wants to move toward normalizing U.S.-Cuba relations but he does not plan to end the U.S. trade embargo imposed against the Caribbean island in 1962. (Reporting by Nelson Acosta and Esteban Israel in Havana and Hilary Burke in Buenos Aires; Editing by Kieran Murray)

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