New evidence emerged of abuse of Iraqi prisoners by US troops, as Iraqi police and witnesses said six people were killed in a bomb blast at a crowded marketplace in Baghdad. The bomb was concealed in a box of bananas in Baya market, western Baghdad, a police officer said, giving a casualty toll of four dead and nine wounded. Witnesses said two more people, both policemen, also died. Elsewhere in Baghdad, US troops and militiamen of Moqtada Sadr traded small arms fire Sunday in Sadr City, the morning after soldiers arrested a top Sadr lieutenant in the district. Troops seized Sayed Amer al-Husseini and at least four other people after 20 US military vehicles sealed off the area round Sadr's office in the deprived district. They fought a gunbattle with the Mehdi Army militia, killing one Sadr supporter and wounding another. Fresh evidence emerged Sunday to belie protests by US President George W. Bush and his administration that abuse of Iraqi prisoners by American troops was confined to a small minority. Bush sought in his weekly Saturday radio address to play down the extent of abuse of Iraqis held at the Abu Ghraib prison near Baghdad, blaming "a small number of American servicemen and women." But one of the seven US soldiers charged with abusing prisoners said she was acting under direct orders from military intelligence to "make it hell" for inmates before interrogation, The Washington Post reported. Sabrina Harman told the Post they were given no rules and little training, and she never read the Geneva Convention until two months after she was charged. Shiite leader Moqtada Sadr is bunkered down in the central shrine city of Najaf as his supporters battle with US forces across central and southern Iraq. On one of the main streets, Sadr supporters had set light to tyres, and militia were deployed around the offices where Husseini was arrested. Also, at least five British soldiers were reported wounded in Amara on Saturday and three militiamen killed, while four soldiers were wounded and two Sadr supporters killed in fighting in the main southern city of Basra. The US military reported Sunday that one of its soldiers was killed and another wounded Saturday in a mortar attack on the coalition base in the main northern city of Mosul. Also, New Yorker magazine on Sunday released a new Iraq prison abuse picture showing a naked prisoner cowering under threat from two US military dogs. The magazine said American soldiers used "military working dogs to frighten and intimidate detainees with threats of attack, and in one instance actually biting a detainee." The magazine said more images from the same scene showing the prisoner on the floor with blood pouring from a wound. The new photo is accompanied by an article which said Defence Secretary Donald Rumsfeld and top military officers sought to keep the prison abuse scandal quiet for several months. Top investigative reporter Seymour Hersh said he had obtained the images of the Abu Ghraib prison which had been in the possession of a member of the 320th military police battalion. Hersh said that other photos show the dogs "straining at their leashes and snarling at the prisoner. "In another take a few minutes later, the Iraqi is lying on the ground, writhing in pain, with a soldier sitting on top of him, knee pressed to his back. Blood is streaming from the inmate's leg," Hersh wrote in his article. "Another photograph is a close-up of the naked prisoner, from his waist to his ankles, lying on the floor. On his right thigh is what appears to be a bite or a deep scratch. There is another larger wound on his left leg covered in blood. **PHOTO CAPTION*** New Yorker magazine photo.