NIAMEY, May 23 (Reuters) - Islamist suicide bombers struck an army barracks and a French-run uranium mine in Niger on Thursday, officials said, killing 21 people and wounding dozens more in attacks that showed militant violence spreading in West Africa.The coordinated dawn assaults on Areva's mine at Arlit and the military base in Agadez were claimed by the MUJWA militant group in retaliation for a French-led offensive this year against Islamist insurgents in neighbouring Mali.The attacks suggested Malian groups, despite the French campaign, remained capable of complex strikes against high-profile targets in remote parts of the vast Sahara.