DUBAI: Bahrain has charged 15 policemen with "mistreatment" of detainees, the government said on Tuesday, as part of an investigation into reports of torture of protesters rounded up in a crackdown on unrest in the Gulf Arab state.

Bahrain's human rights record has come under international criticism since the authorities crushed Shi'ite Muslim-led demonstrations demanding democracy in the Sunni-ruled state that began in February 2011, inspired by other Arab popular revolts.

"The latest complaints were made during the month of June and nine of the complainants have already been questioned, resulting in three of them being referred to forensic doctors," Nawaf Hamza, head of the Public Prosecution's Special Investigation Unit, said in a statement.

"As a result 15 policemen have been questioned and informed of the charges against them. The investigation of the remaining complaints and those involved is ongoing," he said in comments published by the government Information Affairs Authority.

The IAA said earlier that a number of policemen - it did not say how many - accused of mistreatment of detainees and use of excessive force had been sentenced to five years in prison.

In June it cited an investigation of 19 security personnel, including officers, and said that two officers were sentenced to three months in prison.

Although Bahrain has put police officers on trial for abuse and lethal torture, international rights groups and opposition activists say the government has been dodging accountability at higher levels where security policy is decided.

Last month Bahrain's interior minister denied that police had been given any orders to torture or kill protesters.

In pursuing prosecutions over torture, the Gulf Arab state has been responding in part to U.S. calls for justice and reform - sensitive because Manama hosts the U.S. Fifth Fleet and wants to be able to keep buying U.S. weapons for its armed forces.

A commission of international legal experts reported in November that torture had been systematically used to punish and extract confessions from hundreds of protesters during a period of martial law after a crackdown on anti-government protests.

It also said that 35 people, mainly protesters, died during the unrest and that five of them died as a result of torture.

Although Bahraini security forces, backed by Saudi troops, broke up a mass protest camp in Manama in March 2011, police and demonstrators continue to clash almost daily. Each side blames the other for the violence.

Bahrain said last week it had uncovered a large bomb-making operation and named three suspects.

The IAA said in another statement that forensic experts from London's Metropolitan Police were in Bahrain to help investigate that case. It said there could be foreign involvement, pointing at Lebanese Shi'ite Hezbollah militants.

"Bahraini officials have stated that the level of sophistication of the bomb-making operation suggests foreign involvement and may indicate a connection with Hezbollah."