(Adds comment from RCS lawyer, details about arbitration; changes sourcing in first paragraph)
MILAN, April 24 (Reuters) - A New York judge has put on hold a lawsuit by U.S. private equity firm Blackstone Group LP against Italian publisher RCS Mediagroup over the disputed sale of real estate assets, pending the outcome of an arbitration in Italy.
The case centres on RCS' historic headquarters in central Milan, which Blackstone bought from RCS for 120 million euros ($134 million) in 2013. Both companies now claim ownership.
RCS Chairman Urbano Cairo, who became RCS' controlling shareholder after the 2013 deal, has said Blackstone took control of the headquarters at too low a price, while RCS faced financial difficulties, and the transaction should be voided.
Blackstone has in turn accused RCS of falsely claiming that it still owns the property, and improperly blocked it from selling the property to Germany's Allianz SE.
Justice Saliann Scarpulla of the New York state Supreme Court in Manhattan on Wednesday put Blackstone's lawsuit on hold pending the outcome of the Italian arbitration. That process could take one year.
Aaron Marks, a partner at Kirkland & Ellis representing Blackstone, said his client would pursue its claims to the end, whether in New York or Italy.
"We are entirely confident in our legal case and have no intention of settling," Marks said in an emailed statement.
Michael Carlinsky, a partner at Quinn Emanuel Urquhart & Sullivan representing RCS, said: "We are pleased with the court's decision to respect the Italian arbitration and allow it to go forward in Italy."
RCS, the owner of influential daily Corriere della Sera, launched arbitration proceedings in Milan last year to nullify the 2013 sale under Italian law.
The next hearing in that case has been scheduled for Sept. 16, according to two sources close to the matter.
In the New York case, Blackstone had argued that the planned sale to Allianz was held up by claims from RCS that the transaction was "null and void."
The dispute has alarmed some real estate investors, who have said foreign investment in Italy could suffer as a result. ($1 = 0.8963 euros) (Reporting by Claudia Cristoferi and Silvia Aloisi in Milan; additional reporting by Jonathan Stempel in New York; editing by Hugh Lawson and Jonathan Oatis)