The offer price values Celtic at about 161 million pounds ($329 million), 33 percent more than the 115 million pound ($235.7 million) value on Sept. 17, the day before Celtic announced that it had been approached regarding a potential offer.

London-based Celtic, which operates mines in the former Soviet republics, said Severstal's offer continues to focus "on the past."

"It completely ignores Celtic's strong financial position and asset base," the company said in a statement.

"Severstal is trying to buy Celtic cheaply through this opportunistic and unsolicited offer," Celtic Chairman Peter Hannen said.

The Russian company, however, said its bid puts "an attractive premium" on Celtic's market price.

"In the absence of our offer or a competing offer, we believe that the current Celtic Share price is unsustainable, and that the price of Celtic Shares may fall substantially."