* Ousted prime minister in exile in Lisbon after coup
* Caretaker gov't says he ordered killing of former president
BISSAU, Oct 24 (Reuters) - Guinea-Bissau's caretaker government said on Wednesday it sent an extradition request to Portugal for its exiled former prime minister, whom it accuses of ordering the assassination of a former president in 2009.
Tensions between Guinea Bissau and its former colonial ruler Portugal have soared since a military coup in April derailed presidential elections that Portuguese-backed Carlos Gomes Junior was widely favoured to win.
Guinea-Bissau officials accused Portugal of seeking to return Gomes Junior to power, and said Lisbon was behind a failed counter-coup effort over the weekend in which at least six people were killed.
"Carlos Gomes Junior should have the decency to appear in court," Fernando Vaz, a spokesman for Guinea-Bissau's transitional government said on Wednesday, adding Gomes Junior was "implicated in several horrific murders" including that of late president Joao Bernardo Vieira.
He said a formal extradition request was sent to Lisbon on Oct. 1. A Portuguese government official was not immediately available to comment, though the foreign ministry has said it can deny extradition requests to countries that do not meet certain judicial and human rights criteria.
Guinea Bissau's military, led by General Antonio Indjai, detained Gomes Junior in an April 12 coup ahead of a run-off presidential election he was favoured to win.
The election was meant to shore up stability in the impoverished cashew-producer nation, which has earned a reputation as a haven for Latin American drugs cartels ferrying cocaine to Europe.
Indjai accused Gomes Junior of having a secret pact with Angola, which had troops in the country at the time, to eliminate the army leadership.
Gomes Junior was released but forced to leave the country under a deal brokered by West African regional bloc ECOWAS that has since been criticised by the United Nations, the European Union, and the CPLP grouping of Portuguese-speaking nations for rewarding the coup leaders.
The civilian-led transitional government this week accused Portugal and other CPLP members of being behind an attack on a Bissau military base over the weekend.
Gomes Junior's office in Lisbon, which claims to represent the 'legitimate government of Guinea Bissau', issued a statement saying the attack appeared to have been a fabrication to 'whitewash' the failures of the current leadership.
He has said he hopes to return to Bissau to rule. (Reporting by Alberto Dabo; Additional reporting by Andrei Khalip in Lisbon and Shrikesh Laxmidas in Luanda; Writing by Richard Valdmanis; Editing by Michael Roddy)