* Speculation rife again about socialist leader's health
* Chavez had talked about attending meeting in Brazil
* Venezuela just admitted to Mercosur trade bloc
Venezuelan President Hugo Chavez will not attend a regional trade summit in Brazil on Friday, sources said, an absence sure to heighten speculation over the leftist leader's health.
Before he went to Cuba last week for more cancer-related treatment, Chavez had spoken enthusiastically about attending the Mercosur bloc's meeting in Brasilia to celebrate Venezuela's entry.
He has not been seen in public since Nov. 15.
"Chavez is not coming. They've even canceled the hotel reservation," a Brazilian foreign ministry source said on Thursday. Another Brazilian official said Chavez's logistics and security advance staff were leaving Brasilia.
In Caracas, a source at the Miraflores presidential palace also said the 58-year-old Chavez was not going to travel.
The recently re-elected president went to Cuba nine days ago 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 more serious.
"If Chavez does not show up at the first meeting where Venezuela is a full member of Mercosur, it will create a lot of distrust over his health," said Marcelo Coutinho, professor of international relations at Rio de Janeiro's Federal University.
Officials have given no detailed information on Chavez's condition.
SPECULATION AND SCENARIOS
Later on Thursday, Venezuela's information minister tweeted links to two statements by the president, one extolling a group of colorful Venezuelan cultural dancers, Los Danzantes de Corpus Christi, and another expressing his regret at the death of Brazilian architect Oscar Niemeyer on Wednesday.
Neither statement mentioned Chavez's health, his treatment in Cuba, nor his absence from the meeting in Brasilia.
His presence in Cuba, where Chavez has spent more than three months in total since his first 2011 diagnosis of cancer in the pelvic region, has overshadowed Dec. 16 regional elections.
Without him on the campaign trail or television, the race for 23 governorships has been a low-key one, failing to light up the public like October's presidential election did with a record voter turnout of more than 80 percent.
Although Chavez continues to sign official papers and make appointments from Cuba, his absence may delay decisions on some major issues like a possible devaluation of the bolivar currency or an amnesty for jailed opponents.
Various ministers have been visiting him in Havana this week, according to Venezuelan officials.
Chavez allies have lambasted media and others for behaving like "vampires" with speculation about his condition. One pro-opposition journalist said he was using a wheelchair.
Bonds have rallied since his departure, on investor bets for a change to a more business-friendly government.
Newly appointed Vice President Nicolas Maduro, 49 - a former bus driver and union leader - has assumed a more prominent position amid speculation among Venezuelans over who could replace Chavez should he leave office.
Maduro, who is also foreign minister, was due to represent Venezuela at the Mercosur meeting in Brazil.
Congress chief Diosdado Cabello, a former military comrade of Chavez, is also often touted as a possible successor to lead the ruling Socialist Party.
Under the constitution, an election would have to be held if Chavez were to leave office within the first four years of his new six-year term, which starts on Jan. 10.
That would give a fresh chance to the opposition, which garnered 44 percent of the vote - and a record number of 6.5 million votes - in the October presidential poll.
An internal U.N. report seen by Reuters drew three possible scenarios.
"If Chavez lives, his term will continue to face monumental political, economic and security challenges. If Chavez accepts his mortality and organizes a managed transition, a unified PSUV would likely fare well in a snap election," the report said.
"If Chavez dies unexpectedly without a clear successor, a unified opposition - likely behind former presidential candidate Henrique Capriles Radonski - would stand its best chance to come to power in over a decade."
Chavez is guaranteed a warm welcome and discreet treatment because of his friendship with Cuba's past and present rulers Fidel and Raul Castro, plus the strict controls on information there.