Lagos - Nigeria's new president Muhammadu Buhari said on Monday that he will not appoint cabinet ministers until September, having taken office on May 29."Nigeria must first put new rules of conduct and good governance in place," Buhari wrote in an article published in the Washington Post on Monday. In the article, he outlined his plan for defeating the militant Islamist group Boko Haram and rooting out corruption.Boko Haram has killed thousands of people in a six-year insurgency, during which it has attempted to set up a state adhering to strict sharia law in the northeast of Nigeria, Africa's largest economy.Buhari held talks with US President Barack Obama at the White House on Monday as part of a four-day visit to Washington. Obama said Nigeria's president had a "clear agenda" for defeating the insurgents.Since taking office, Buhari has replaced his defence chiefs and dissolved the board of the state oil company but is yet to appoint a cabinet, prompting questions from some in the Nigerian media."When cabinet ministers are appointed in September, it will be some months after I took the oath of office," wrote Buhari."It would neither be prudent nor serve the interests of sound government to have made these appointments immediately on my elevation to the presidency," he said.