Two sources close to Vekselberg told Reuters last year that he planed to buy a 12 percent stake in Rusal from Prokhorov's Onexim Group via SUAL Partners.

Newspaper Vedomosti reported in 2016 that the deal may be worth $700 million and was due to be finalised by the end of November.

Asked about the deal in the middle of December, Vekselberg told Reuters there were "some certain technical questions which need to be solved", adding he hoped the deal would be closed in the near future.

The sources did not specify on Monday why the talks had stalled.

As one possible reason, Vedomosti quoted two sources as saying on Sunday that Rusal's value had almost doubled since the summer when the talks started.

Prokhorov's Onexim owns a 17 percent stake in Rusal. SUAL Partners, controlled by Vekselberg and his partner Leonard Blavatnik, own another 15.80 percent in Rusal.

The biggest Rusal shareholder is En+, a holding company of Russian businessman Oleg Deripaska.

Rusal, En+ and Onexim declined to comment. A spokesman for Vekselberg's Renova investment vehicle said the company does not comment on rumours.

(Reporting by Anastasia Lyrchikova, Polina Devitt, Andrey Kuzmin and Katya Golubkova; Editing by Keith Weir)