Baghdad - At least four people were injured Wednesday in a bomb blast on a bus in north Baghdad while the US military said it captured a suspected Iranian-trained explosives expert in Kut in southern Iraq.

A bomb went off on a bus in the Shiite-dominated Kazimiyah in north Baghdad, injuring four passengers, police sources told the Voices of Iraq news agency.

US troops arrested on Wednesday a suspected special group explosives expert in Numaniyah, 180 kilometres south-east of Baghdad near the city of Kut, according to a US military statement.

The military uses the term "special groups" to describe Shiite fighters breaching a truce order issued by radical cleric Moqtada al-Sadr, whose militia fought Iraqi and US troops for months until the truce was reached in May.

Acting on intelligence from other Shiite militiamen already in detention, US soldiers captured the expert, who is believed to have travelled to Iran several times for training, the military said.

Troops entered the suspect's home and subdued him without firing any shots, according to the statement.

The Iranian government denies long-standing US allegations that Tehran arms, funds and trains Shiite militiamen in Iraq.

Iraqi Prime Minister Nuri al-Maliki, himself a Shiite, discussed during a visit to Tehran last weekend the US arrests of fighters allegedly trained by Iran. (dpa)