BRASILIA (Reuters) - Venezuelan President Hugo Chavez looks set to miss a regional trade summit in Brazil on Friday after going to Cuba for cancer-related medical treatment, officials said.

Chavez had been enthusiastic about attending the Mercosur bloc's meeting in Brasilia to celebrate Venezuela's entry, so his absence will fuel speculation about his health.

"It looks like he is not coming, but the Venezuelans have not yet confirmed that," Brazilian foreign ministry spokesman Tovar da Silva Nunes said.

In Caracas, a source at the Miraflores presidential palace also said Chavez, 58, was not going to travel.

The recently re-elected socialist president has not been seen in public for three weeks, and last week traveled to Cuba for "hyperbaric oxygenation" treatment - normally used to treat bone decay caused by radiation therapy.

Chavez has had three cancer surgeries in Cuba since mid-2011, so even though officials were portraying the latest treatment as normal follow-up after radiation, rumors are rampant that it could be a more serious downturn.

Officials have given no detailed information about the condition of Chavez, who has been in Cuba for nine days.

(Reporting by Anthony Boadle in Brasilia, Deisy Buitrago in Caracas; Writing by Andrew Cawthorne; Editing by Doina Chiacu)