MAIDUGURI (Reuters) A militant group claimed responsibility for a bomb attack on the UN headquarters in Nigeria that killed 23 people, demanding the release of prisoners and an end to a security crackdown to prevent further bombings. Boko Haram was the prime suspect of Friday's car bomb on the United Nations' Abuja office - one of deadliest attacks on the world body in its history. Meanwhile, clashes broke out between Christians and Muslims in tense central Nigeria on Monday, with witnesses reporting a number of deaths as well as vehicles and at least two shops set ablaze.