Seven men have been arrested in the city of Basra, including three thought to be senior or middle ranking officers.
The British military has said the officers were suspected of involvement in the recent killing of 17 employees of a British-run police training academy.
Earlier, the US Secretary of State, Condoleezza Rice, again defended the war in Iraq, saying the investment in US lives and dollars will be 'worth it'.
Dr Rice acknowledged a lot had been sacrificed in Iraq but once it emerges as a country that is a stabilising factor, there will be a very different kind of Middle East.
She was speaking shortly after eight US marines were charged over the deaths of 24 Iraq civilians in Haditha last year.
Iraqi Prime Minister Nuri al-Maliki described the incident as a 'terrible crime'.
Iraqi witnesses say marines shot the civilians in their homes to retaliate for the death of a comrade who died nearby in a roadside bomb.
Following a three-day visit to Iraq, US Defence Secretary Robert Gates said the US and the Iraqi government were in 'broad strategic agreement' on how to bring security to Baghdad.
Mr Gates also confirmed that the US naval presence in the region had increased in recent weeks, but he would not comment on claims that a second aircraft carrier was being sent to the Gulf.
