Italian prosecutors are investigating Royal Dutch Shell Plc (NYSE: RDS.A) over allegations of international corruption in relation to a big Nigeria oil deal that also involved rival Eni, a judicial source said on March 30.

Milan prosecutors opened a corruption probe into Eni SpA (NYSE: E) in 2014 in a case relating to a $1.3 billion acquisition of Nigeria's OPL-245 offshore oil block in 2011 by the Italian company and Shell.

They later placed under investigation Eni's Chief Executive Claudio Descalzi and another top manager at the company. The probe has now been widened to Shell, the source said, confirming a report in Corriere della Sera daily on March 30.

Shell said on March 30 Dutch investigators recently visited its headquarters in the Hague in the Netherlands in relation to an investigation into a Nigerian offshore oil field.

"Representatives of the Dutch Financial Intelligence and Investigation Service and the Dutch Public Prosecutor recently visited Shell at its headquarters," a spokesman said.

"The visit was related to OPL 245, an offshore block in Nigeria that was the subject of a series of long-standing disputes with the Federal Government of Nigeria," the spokesman said.

Shell is cooperating with the authorities and is looking into the allegations, the spokesman said.

Eni has denied any wrongdoing in the case.