The United States, Britain and the European Union expressed concern on Saturday after Nigeria's president suspended the country's chief justice three weeks before the presidential election, with the US warning it could "cast a pall" over the vote in Africa's most populous nation.President Muhammadu Buhari set off an uproar on Friday by announcing the suspension, citing corruption allegations. The chief justice would play a key role in any legal challenge to the election in which Buhari seeks a second term.The US said Buhari acted "without the support of the legislative branch" and noted widespread criticism in Nigeria that the move was unconstitutional. It urged Nigerian authorities to quickly resolve the crisis that could undermine the credibility of the February 16 vote.At stake is a country that is Africa's largest oil producer, with a population of some 190 million and multiple security challenges, including the decade-old Boko Haram extremist insurgency. Buhari's election in 2015 was a rare peaceful transfer of power.Britain in a separate statement said that "we are compelled to observe that the timing of this action, so close to national elections, gives cause for concern. It risks affecting both domestic and international perceptions on the credibility of the forthcoming elections."The EU election observer mission called on all parties to "respond calmly."Some Nigerian elections have been marked by violence.The Nigerian Bar Association has called Buhari's move an "attempted coup against the Nigerian judiciary." Senate president Abubakar Bukola Saraki has said the president in such a matter cannot act alone.Buhari's top election challenger, former vice president Atiku Abubakar, has called the suspension illegal and "an act of dictatorship."Nigerians have pointed out that the suspension occurred shortly before the chief justice was to swear in members of various election petition tribunals.The chief justice, Walter Nkanu Samuel Onnoghen, faces trial on charges of allegedly failing to declare his assets. Buhari said his suspension will continue until the case is concluded. This is the first time a chief justice is standing trial in Nigeria, where corruption is widespread. Onnoghen's side has argued that the charges lack merit.Beyond the charges, "security agencies have since then traced other suspicious transactions running into millions of dollars" to Onnoghen's accounts, the president said in a statement.Justice Ibrahim Tanko Muhammed will act as Nigeria's most senior judge, Buhari said. Muhammed, like the president, is from Nigeria's largely Muslim north while Onnoghen is from the largely Christian south.Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.