Romania's Prime Minister Victor Ponta resigned Sunday as head of the Social Democrat party as he battles corruption charges that have prompted calls for him to step down as government leader.

Ponta has been under investigation since last month for money laundering and tax evasion, but has refused to resign his post as premier citing his duty "to millions of Romanians."

"In order not to let this situation affect the PSD (Social Democrats), I have decided to suspend my leadership role in the party until I have proven my innocence," Ponta wrote on Facebook.

Ponta returned to work on Thursday following a month-long stay in Turkey for a knee operation.

He had left the country a little more than a week after coming under investigation for fraud, tax evasion and money laundering dating to 2007-11, before he became prime minister in 2012.

Separately, prosecutors suspect he is guilty of conflict of interest as premier. But that probe was stymied when parliament, where Ponta's party has a comfortable majority, refused last month to lift his immunity from prosecution.

The centre-left prime minister's legal troubles prompted a fresh crisis in the European Union's second poorest nation, where President Klaus Iohannis and the opposition has called on Ponta to step down.

Ponta has firmly denied the allegations and has promised to cooperate fully with prosecutors.

The money laundering and tax evasion allegations relate to his activities as a lawyer.

Ponta is accused of receiving the equivalent of around 55,000 euros ($61,000) from Dan Sova, a political ally and member of parliament suspected by prosecutors of abuse of power but who also enjoys immunity.

This probe is the latest in a string of enquiries by the anti-corruption directorate (DNA) that have cost several prominent Romanians their jobs in recent months