Sudan's President Omar al-Bashir has left Nigeria amid calls for his arrest on charges of genocide in Darfur.
A human rights group on Monday filed a case in a Nigerian court to try to compel the government to arrest Mr Bashir and hand him over to the International Criminal Court (ICC).
Mr Bashir was in Nigeria for an African Union-organised health summit due to end on Tuesday.
Sudanese diplomats said he left because he had another engagement.
The ICC issued an arrest warrant for Mr Bashir in 2009, accusing him of committing genocide during the 10-year conflict in Sudan's Darfur region.
On Tuesday, the ICC issued a statement saying it had asked Nigeria to arrest Mr Bashir before he left the country.
Sudan does not recognise the ICC and accuses it of being a tool of Western powers, while the AU has called on its members not to arrest Mr Bashir.
Mr Bashir was to due to speak at the summit in the capital, Abuja, on Monday, Nigeria's Guardian newspaper reports.
But when he was called to make a presentation, he was nowhere to be found, it says.
Sudan's ambassador to the African Union Abdelrahman Sirelkhatim Mohamed denied Mr Bashir left to evade arrest, AFP news agency reports.
"He's the bravest," Mr Mohamed is quoted as saying.
"If he's afraid of arrest, he would not have come here."
Mr Bashir received a full guard of honour from the Nigerian government when he arrived in Abuja on Sunday to attend the summit, which is looking at ways to curb malaria, Aids and tuberculosis in Africa.
He was among eight African leaders who attended the summit, Associated Press news agency reports.
Some had left on Monday, but others, including Kenya's President Uhuru Kenyatta, were still at the summit on Tuesday, Reuters news agency reports.
Nigerian presidential spokesman Reuben Abati told AP that Mr Bashir had been in Abuja at the AU's invitation, not Nigeria's.
Nigeria allowed him into the country in accordance with an AU decision not to cooperate with the ICC, he said.
The Nigerian Coalition for the International Criminal Court (NCICC) filed papers in the High Court on Monday, to push the government to arrest Mr Bashir.
Nigeria was in breach of its international obligations by failing to arrest him, and was fuelling a culture of impunity, NCICC chair Chino Obiagwu said.
Sudan's foreign ministry accused the UK of being hypocritical by criticising Nigeria for failing to arrest Mr Bashir.
"Britain participated in the Iraq invasion after it had manipulated the domestic and international opinion with reasons it knew were lies. Iraq, our friend, still suffers from the destruction," the ministry said in a statement, Reuters reports.
On Monday, the UK minister for Africa, Mark Simmonds, said Nigeria's decision to host Mr Bashir "undermines the work of the ICC and sends the victims a dismaying message that the accountability they are waiting for will be delayed further".
Although Nigeria's government has resisted the calls to arrest Mr Bashir, in 2003, it handed ex-Liberian President Charles Taylor to a UN-backed court to stand trial on war crimes charges, following intense diplomatic pressure from the US.
Mr Taylor had been exiled in Nigeria, and was arrested as he tried to flee.
Mr Bashir has visited numerous African countries since the arrest warrant was issued - including Ethiopia, Kenya and Malawi.
Last year, Malawi's new leader said she did not want Mr Bashir to attend a summit there, reversing the position of her predecessor.
In 2011, a Kenyan court ruled that Sudan's president should be arrested if he ever visited the country following a case brought by a non-governmental organisation, the International Commission of Jurists (ICJ).
Some 2.7 million people have fled their homes since the conflict began in Darfur in 2003, and the UN says about 300,000 have died - mostly from disease.
The ICC says Mr Bashir's government backed Arab militias who targeted civilian members of the Fur, Masalit and Zaghawa communities.
Sudan's government says the conflict has killed about 12,000 people and the number of dead has been exaggerated for political reasons.