Top Ugandan rebel commander Dominic Ongwen is being flown toThe Hague to stand trial on war crimes charges.
Mr Ongwen, a feared commander of the Lord's Resistance Army (LRA), was taken into US custody in the Central African Republic (CAR) earlier this month.
Uganda agreed that he should be tried by the International Criminal Court (ICC), despite being a fierce critic of The Hague-based court.
US and African forces had been searching for Mr Ongwen since 2011.
He is said to be the deputy to LRA commander Joseph Kony, who is still on the run.
The ICC said in a statement that Mr Ongwen would be held in a detention centre in the Netherlands until his trial.
"His flight took off around 1700 GMT and he should arrive tomorrow morning," ICC spokesman Fadi El Abdallah told AFP news agency.
Mr Ongwen would undergo a medical visit upon his arrival and a date would soon be set for his first court appearance, the statement added.
The US is not a signatory to the Rome Statute that set up the ICC, but it facilitated his transfer to the ICC.
It offered a $5m (£3.3m) reward in 2013 for information leading to Mr Ongwen's arrest.
He is accused of war crimes and crimes against humanity, including enslavement.