DHAKA, June 14, 2009 (AFP) - A top United States diplomat Sunday called for a "credible investigation" into the deaths of 21 Bangladesh border guards held in custody after a mutiny in which scores of army officers were slaughtered.

Assistant Secretary for South and Central Asian Affairs Robert Blake said he discussed the issue of the deaths and the mutiny with top Bangladeshi officials during his two-day visit to the country.

"It's very important for the government to investigate, to carry out a credible investigation and to bring those who might have been responsible for the deaths in custody to justice," said Blake.

Bangladesh has said four of the 21 committed suicide and last month ordered a probe to determine the causes of the "unnatural deaths".

At least 74 people were killed in the revolt on February 25 and 26, including 57 senior army officers who were gunned down and had their bodies dumped in sewers and shallow graves.

Police have detained more than 3,000 border guards and questioned thousands of others on mutiny charges, which raised fears for the survival of the country's newly-elected government.

Blake, who will meet Bangladesh Prime Minister Sheikh Hasina later Sunday before departing, said the US would help reconstitute Bangladesh border guards provided a proper investigation was conducted into the deaths.

"Those kind of actions will help provide the basis for increased cooperation with the Bangladeshi Rifles as they reconstitute themselves after the incident took place," he said.

Blake on Saturday met ex-premier Khaleda Zia, the leader of the opposition, which continues to boycott the parliament, since it went into session in late January, and urged her to join the house for "constructive democracy".