ONITSHA, Nigeria (Reuters) - A group campaigning for the secession of a part of southeastern Nigeria, formerly known as Biafra, on Tuesday accused the army of laying siege to their leader s home, a charge the armed forces denied.  Rising tensions prompted the governor of Abia state, where the leader s residence is located, to impose a curfew.  Members of the Indigenous People of Biafra (IPOB) group said soldiers had surrounded the home of leader Nnamdi Kanu. Groups have stepped up calls for secession since Kanu was released on bail in April after being detained for nearly two years on charges of criminal conspiracy and belonging to an illegal society.  There was no surrounding of Nnamdi Kanu s residence. It is not true,  said army spokesman Sani Usman.  Secessionist sentiment has simmered in the region since the Biafra separatist rebellion tipped Africa s most populous country into a civil war in 1967-70 that killed an estimated one million people. The military presence in southeastern Nigeria has increased in the last few weeks to crack down on crime. The IPOB also said that soldiers stormed Kanu s family compound on Sunday, which the army also denied.  Politicians waded into the dispute on Tuesday. Abia state governor Okezie Ikpeazu said in a statement that people were advised to observe a curfew from 6 p.m. (1700 GMT) to 6 a.m. (0500 GMT) from Sept. 12 to Sept. 14.  A caucus of southeastern lawmakers in the Senate, the upper chamber of parliament, said in a statement through its chairman Enyinnaya Abaribe that the military had sent a  strong signal that the region is under siege, which should not be so in a democracy . Renewed calls for Biafran secession prompted President Muhammadu Buhari to use his first speech after returning from three months of medical leave in Britain, in August, to say Nigeria s unity was  not negotiable . Amnesty International in 2016 accused Nigeria s security forces of killing at least 150 Biafra separatists at peaceful rallies. The military and police denied the allegations. 