Explosions and fierce clashes have broken out in the Iraqi city of Najaf between US occupation forces and supporters of Shia leader Muqtada al-Sadr. Aljazeera's correspondent reports that six US tanks have been positioned in the Thawrat al-Eishrin square near a police station, since Thursday night. More tanks entered the square on Friday morning. Witnesses told Aljazeera of a heavy exchange of gunfire erupting between the two forces. Citizens, fearing a deteriorating situation, have evacuated the area. Other reports said at least three US tanks were seen in a cemetery about one kilometre from the holy Imam Ali shrine and fighters were seen fanning out across the area. Heavy black smoke was seen rising from the cemetery and the sound of heavy guns was heard. US helicopters also hovered over the area as armed black-clad men, their heads in scarves were seen running inside the sprawling cemetery, northwest of the city centre. Gunfire and loud explosions were also heard coming from an area south of Najaf known as Bahr al-Najaf, where about 2500 US soldiers are camped in the desert. **Karbala*** Al-Sadr's fighters have dug in with heavy weapons on a hill overlooking Bahr, about 100 metres south of the shrine. Their military command post is also in this area right behind the shrine. The fighting comes one day after Najaf's new police chief Ghalib al-Jazairi accused fighters of "terrorising" residents and asked them to leave. Residents reported intermittent gunfire on Friday in the southern city of Karbala, where American troops and the cleric's al-Mahdi Army, have fought intense battles this week. Northeast of Baghdad, a bomb exploded in front of the house of a tribal chief, killing him and one of his daughters. The motive for the killing in Muqdadiyah of Rukan Mughir, head of the Miahi tribe, was unclear. However, residents said he had been cooperating with American occupation forces. Another daughter and a son of the chief were injured in the blast, hospital officials said. **PHOTO CAPTION*** An armed Iraqi loyal to cleric Moqtada Sadr fires at US forces in the heart of Karbala.(AFP)