UPDATE 1-Russia's Novolipetsk Steel faces antitrust fines

(Adds company response, fines in similar case)

MOSCOW, Feb 15 (Reuters) - Russia's anti-monopoly service on Friday said it would not to allow Novolipetsk Steel (NLMKq.L: Quote, Profile, Research) to increase its stake in an indebted peer to 100 percent, and said it might fine Novolipetsk for its acquisition of the firm.

The service said Novolipetsk, Russia's fourth-largest steel maker, did not inform it before buying a controlling stake of 50 percent plus one share in Maxi Group in December.

"The deal was one of the biggest in ferrous metallurgy last year...Performing such big deals without the anti-monopoly service's approval is absolutely inadmissible," Alexei Ulyanov, head of the service's industry monitoring unit, said in a statement.

"We are considering further actions towards the company, such as fines," he said

Novolipetsk, or NLMK, paid $600 million for the controlling stake in Maxi Group, which produces 2.4 million tonnes per year, but has said it could not refinance its debt of $1.8 billion due to the global liquidity crisis.

It said in on Friday that it would not contest any fines the service may impose.

In a similar case last year, gas export monopoly Gazprom (GAZP.MM: Quote, Profile, Research) was ordered to pay $20,000 when it failed to get regulatory approval for the acquisition of a petrochemical firm Salavatnefteorgsintez SNOZ.RTS.

NLMK also said it would re-file its request to build a 100 percent stake in Maxi Group after formally informing the regulator that it already owns 50 percent plus one share.