FILE PHOTO - The ticker and trading information for Blackstone Group is displayed at the post where it is traded on the floor of the New York Stock Exchange (NYSE) April 4, 2016. REUTERS/Brendan McDermid

MILAN (Reuters) - RCS Mediagroup RCSM.MI contested the sale of real estate assets to Blackstone BX.N because it believes the U.S. private equity fund paid too little for them in 2013, a source at the Italian media group said on Wednesday.

The Italian publisher said late on Tuesday that it had launched arbitration proceedings under Italian law last week to annul the sale of the properties in central Milan that it had sold to Blackstone. It gave no further details in its statement.

RCS, the owner of influential daily Corriere della Sera, believes Blackstone paid too little for the properties because the media group was in financial difficulties at the time of the sale, the RCS source said.

The publisher contested the sale through a letter in March and then again in July before starting arbitration proceedings last week, the source said.

A source close to Blackstone said the private equity group believed that RCS’s attempt to void the sale was “wholly fabricated”.

RCS Mediagroup could not immediately be reached for comment.

Blackstone launched a separate legal case against RCS Mediagroup in New York on Tuesday, alleging a planned sale of the same properties to Allianz ALVG.DE was held up by claims from RCS that the transaction was "null and void", according to a copy of the court documents seen by Reuters.