BURUNDI (REUTER) - In the worst massacre, the U. N. statement said Burundian troops killed about 700 people in northern Kayanza region on December 2-3. Another big massacre on December 13 in the rural region of capital Bujumbura claimed 121 lives, it said. "The human rights situation in Burundi was characterised by a new and more serious outburst of violence and military operations in the reporting period," it said. U.N. human rights observers were also informed of rebel killings of some 58 people in the same period, it added. Diplomats and aid workers say the pace of massacres has picked up since the army grabbed power in a coup in June. The U.N. says the army has also killed hundreds of Hutu refugees who returned home from eastern Zaire late in 1996. The army denies it is killing Hutus.