Nigerian President Muhammadu Buhari ended a four-day visit to the Netherlands on Wednesday, after voicing staunch backing for the International Criminal Court and giving oil giant Shell a nod for a new $5bn investment deal."The take away from the president's visit was very positive," Nigeria's Foreign Minister Geoffrey Onyeama told AFP shortly after Buhari's jet took off from Rotterdam Airport.Buhari was the only global president invited by the ICC, the world's court of last resort, to address a ceremony marking the 20th anniversary of its founding Rome Statute.He agreed to make the keynote speech at the event, despite calls from some African leaders for a mass withdrawal from the court in The Hague, amid allegations of bias against the continent."Nigeria made it very clear that the ICC is a very important institution... We never saw the issue of being biased against African leaders as proven and necessarily right," said Onyeama.Serious crimes "Our position was that if there are any faults or whatever, then those faults can always be addressed from within the ICC and not with the mass withdrawal because the main objective of the organisation is a good one," Onyeama said.In 2017, under investigation for suspected crimes against humanity in which 1 200 people were said to have died, Burundi became the first country to withdraw from the tribunal.But even though the ICC launched a preliminary investigation in 2010 into possible atrocities by both the Islamist Boko Haram militant group and Nigerian security services, Buhari insisted the tribunal was a "vital global institution"."With the alarming proliferation of the most serious crimes around the world, the ICC, and all that it stands for, is now needed more than ever," he said on Tuesday.Buhari also held talks with the Anglo-Dutch oil giant Shell, and gave the company the nod for a massive investment in an oil and gas project.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookHe "met with the Shell CEO and has more or less given the greenlight and encouragement for Shell to sign the over five billion dollar investment project for Nigeria in the gas and oil fields," said Onyeama, without giving further details."So that is a huge amount of money to be investing and we really need those kind of investment in the country at the moment." Buhari also held bilateral talks with the Dutch Prime Minister Mark Rutte to discuss issues including the insurgency in Nigeria's north east, climate change and the shrinkage of Lake Chad and graft.The two countries also signed a memorandum of understanding to set up a mechanism to improve trade. Nigeria's official statistics put trade volume with the Netherlands at about $3.5bn in the first quarter of 2018, making the Dutch the largest trading partner of Africa's most populous nation.Buhari, who is seeking re-election next year, has urged greater diversification to counter his country's over-riding reliance on oil revenues. He also met dozens of company chief executives to encourage greater Dutch investment.Maikanti Baru, managing director of the state oil company, Nigerian National Petroleum Corporation (NNPC), told AFP that Buhari's meetings with Dutch CEOs "have shown that they have a lot of confidence in the Nigerian economy."He added in total Shell had already committed to bringing in $15bn to the Nigerian economy in the next few years.