By Elias Biryabarema

KAMPALA (Reuters) - Uganda's military said on Wednesday it was investigating an army officer who is an aide to President Yoweri Museveni's brother on charges that he was involved in securing a payment worth 500,000 euros from a foreign firm using a fake procurement contract.

Businesses often complain about corruption in Uganda as a hindrance to doing business, while the opposition says graft also pervades the army. The government has vowed to crack down on corruption but top officials are rarely convicted.

Military spokesman Paddy Ankuda told a news conference that Captain Ronald Mutebi Muhoozi, an aide to Museveni's brother Salim Saleh, had been detained alongside a civilian suspect for questioning over a corruption case.

Salim is a retired senior officer who has an unofficial role as an adviser to President Museveni, who came to power in 1986 after leading a rebel group.

Lawyers for Muhoozi and his civilian accomplice, named as Sam Ssimbwa, could not immediately be reached for comment.

Ankunda said last year that the army captain and his civilian accomplice had colluded with others and posed as Defence Ministry representatives over a fake deal to procure body armor and armored vehicles.

As part of the deal, the foreign firm was asked to pay an estimated 500,000 euros to "help in the processing of documentation," Ankunda said, adding the Ugandans behind the contract disappeared after that sum was paid.

Ankunda said the purported procurement was "pure fraud" and that the military had never issued such a tender. "We're interested in finding out who pocketed this money," he said.

(Reporting by Elias Biryabarema; Editing by Edmund Blair)