Romania's prime minister says he has undergone a "complicated" knee operation abroad following questions about his whereabouts.

Victor Ponta, who is a suspect in a corruption probe, previously announced he had left deputy Prime Minister Gabriel Oprea in charge until Friday, without saying why he was not in Bucharest.

On Tuesday Ponta said on his Facebook page he had had knee surgery abroad.

President Klaus Iohannis criticized Ponta for not letting him know where he was and for attending the opening ceremony of the European Games in Baku on Friday, an event that most European Union leaders avoided due to concerns over Azerbaijan's human rights record.

Prosecutors said Ponta is suspected of money-laundering, forgery, tax evasion and conflict of interest for work he did as a lawyer in 2007-2008.