Nigeria offensive pushes 6,000 refugees into Niger: U.N. LAGOS (Reuters) - Nigeria's military offensive against Islamist militants in its northeast has forced more than 6,000 refugees - mostly women, children and the elderly - to flee to neighboring Niger, the U.N. refugee agency (UNHCR) said on Tuesday. Nigerian forces are engaged in a 4-week-old operation to regain territory from fighters loyal to Islamist group Boko Haram, which they say has enabled them to wrest back control of the country's remote northeast. They also say they have destroyed key bases and arrested more than 150 suspected insurgents in the states of Borno, Yobe and Adamawa -- all covered by a state of emergency declared by President Goodluck Jonathan last month. Islamist groups like Boko Haram and the al Qaeda-linked Ansaru have become the biggest risk to stability in Nigeria, Africa's top oil producer and second largest economy, and some Nigerians saw the military push as long overdue. But rights groups and aid agencies fear the longer it goes on, the more the region's vulnerable local population, which includes some of the poorest people on earth, will suffer. "Those UNHCR has spoken to say they escaped for fear of being caught in the government-led crackdown," notes from a UNHCR briefing in Geneva said, adding that they had reported air strikes were continuing near Lake Chad, where Nigeria borders Chad, Cameroon and Niger. The military was not immediately available for comment, but in a statement on Friday defense spokesman Brigadier-General Chris Olukolade denied a report that Nigerian refugees were "pouring into" Niger. The UNHCR said Niger had received 2,692 Nigerian citizens, with another 3,544 originally from Niger but who were living in Nigeria, and 94 people of other nationalities, mainly Chadian. They are "putting a strain on meager local food and water resources. Niger ... itself struggles with food insecurity due to years of drought," the U.N. agency said. Separately, two Boko Haram suspects were shot dead on Sunday in the Gwange area of the northeast while trying to escape detention, two witnesses and a military source said. Boko Haram militants have dispersed since the raids began, raising fears they could re-arm and return. Reporting by Tim Cocks; Additional reporting by Lanre Ola in Maiduguri; editing by Mike Collett-White