Abuja - Nigeria's Senate on Wednesday rejected the appointment of President Muhammadu Buhari's nominee to head the country's anti-graft agency, in a blow to the government's war on corruption. Buhari rode to power in 2015 on an anti-corruption platform and appointed Ibrahim Magu as acting chairman of the Economic and Financial Crimes Commission (EFCC). But the Senate refused to approve Magu's official appointment, citing internal security reports that accuse him of corruption. "Magu has failed the integrity test and will eventually constitute a liability to the anti-corruption stance of the current government," Senator Dino Melaye said, quoting from a report by the Department of State Services.Buhari had previously cleared Magu after investigating the allegations in the reports, which have not been made public. Anti-corruption campaigners say that the politicisation of Magu's appointment - what is normally a routine approval - highlights how difficult it is to tackle endemic graft in Nigeria, one of Africa's top oil and gas producers."The current chairman is leaving no stone unturned," said Debo Adeniran of the Lagos-based Coalition Against Corrupt Leaders, explaining that many senate members are being probed in connection with graft."They don't want him (Magu) to complete all the investigations that are going on," Adeniran told AFP. "Basically, it's a vindictive approach that the Senate is carrying out," he said, adding "it's demoralising."Buhari was elected two years ago vowing to stop the plunder of state funds by corrupt politicians and public officials in the country of 180 million people.The main opposition party, however, has accused him of conducting a political witch-hunt because many of those arrested and charged were opposition party members or served in the previous administration.