Russian metallurgical and mining company Norilsk Nickel has revised up its offer to buy back 20 percent of its own shares from aluminum giant UC Rusal for $12.8bn, Norilsk Nickel announced today. The offer is valid until 6:00 p.m. Moscow time on March 4. In its previous offer, Norilsk Nickel sought to buy back 25 percent of its shares from UC Rusal for $12bn. One of the terms of the current offer is that UC Rusal, UC Rusal Investment Management and Norilsk Nickel should sign a two-year agreement regulating the procedure for voting with the remainder of Norilsk Nickel's shares held by UC Rusal Investment Management.