Nigeria's President Muhammadu Buhari and a number of opposition leaders on Tuesday signed a deal for peaceful elections but the main challenger for the country's top deal was not present.The 75-year-old former military ruler was joined by representatives of dozens of other parties to commit to what he said should be "peaceful and credible" elections."We, the candidates pledge to uphold the highest standard of conduct and encourage our teeming supporters to do the same," he added at the event in the capital, Abuja.Security around polling is a recurrent concern in Nigeria, where there has been a history of violence stoked by identity politics, ethnicity and religion.In 2011, some 1 000 people were killed after clashes between supporters of Buhari after his defeat to Goodluck Jonathan.The text of the peace accord is the same as that signed before the 2015 polls, which saw Buhari win.But there was no sign of former vice-president Atiku Abubakar, from the Peoples Democratic Party (PDP), who is expected to be Buhari's main challenger at the February 16 polls.Also absent was Donald Duke, a former governor of Cross River state, and Oby Ezekwesili, who was education minister under Olusegun Obasanjo and is an ex-World Bank vice-president.Both are standing for minor parties.Asked about Abubakar's absence, his media advisor said he was "not invited". "Atiku didn't get any invitation to the event," he told AFP.But Abdulsalami Abubakar, another former military ruler and the current chairperson of the National Peace Committee, denied any snub."Everybody was invited. What reason do we have not to invite somebody?" he said."This peace accord was for all political parties to sign to be of goodwill and good behaviour during the elections."At the last election, Jonathan, Buhari and other presidential candidates also signed a pledge of non-violence in the presence of former UN secretary-general Kofi Annan.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.