Violence broke out in Sri Lanka as Norwegian negotiators arrived to try and save the countries disintegrating peace process. Unidentified assailants hurled at least four grenades at a Tamil Tiger rebel office in the island's volatile east on Friday, officials said. The grenades were thrown in through the window and open door of the one-story building in Batticaloa, 220 kilometers (135 miles) east of the capital, Colombo, said Rohan Abeywardene, eastern region police chief. There were no reported casualties but a part of the office roof had collapsed, he said. The attack took place as Vidar Helgesen, Norway's deputy foreign minister, flew by helicopter to the northern rebel-held town of Kilinochchi and began talks with S. P. Thamilselvan, the Tigers' political chief. Norway has played a pivotal role in Sri Lanka's peace efforts, brokering a cease-fire between the government and the Tigers three years ago that halted the island's two-decade civil war. Scores have been killed in the violence, including security forces, rebels and civilians since a top rebel commander split from the mainstream group last year. Abeywardene blamed Friday's attacks on factional fighting among the rebels. Residents in Sri Lanka's Tamil heartland were forbidden from leaving their homes Friday and the main road was closed, a day after a mob of angry Tamils hacked a top police officer to death and threw stones at military vehicles. The curfew, imposed late Thursday on the entire Jaffna Peninsula, the home of most of the island's 3.2 million Tamils, was expected to be lifted later Friday, said police spokesman Reins Pourer.