By Joe Bavier

KINSHASA, June 16 (Reuters) - Congolese opposition leader Jean-Pierre Bemba hopes to return from self-imposed exile in Portugal next month after Senate leaders allowed him to extend a medical leave of absence, one of his advisers said on Saturday.

Bemba, elected a senator after losing landmark presidential elections last year, was granted a 60-day leave by the Senate in early April to seek medical treatment for an old injury.

His departure from Congo followed heavy fighting between his personal guard and government troops in Kinshasa.

The deadline for his return expired last weekend but the Senate leadership said in a statement late on Friday they had extended his leave until July 31 after receiving a letter from him asking to prolong his stay in Portugal.

"(Bemba) informed the Senate of his intention to return at the end of July, as long as his security situation is sorted out," Fidel Babala, one of Bemba's advisers, told Reuters.

The former rebel chief left Democratic Republic of Congo on April 11 after nearly three weeks holed up in the South African embassy, saying troops loyal to President Joseph Kabila were trying to kill him. The fighting killed hundreds of people.

Last week his Movement for the Liberation of Congo (MLC) said the necessary measures were not yet in place to assure his security. His refusal to return had raised the possibility he might lose his Senate seat and the immunity that comes with it.

Bemba, a former warlord, lost to Kabila in a presidential election last October -- the first free polls held in the vast former Belgian colony in more than four decades.

Kabila and his government have placed blame for the violence earlier this year and a subsequent wave of looting on Bemba.

Public Prosecutor Tshimanga Mukeba wrote to the Senate in April requesting Bemba's immunity be lifted so that he could be prosecuted as the "intellectual author" of the violence.

Mukeba told Reuters on Saturday there had been no change in the government's plans to put Bemba on trial but that he was waiting for a response from parliament.

According to documents seen by Reuters, Bemba is accused of threatening state security, murder, armed robbery and destruction of property during the fighting, which erupted in March after his fighters refused to report for demobilisation.

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