KIEV, September 29 (RIA Novosti) - The Ukrainian government auctioned off a strategic Black Sea chemical plant on Tuesday, but invalidated the $634 mln offer just minutes later, claiming a bidders' plot to bring prices down.

The auction to sell the Odessa Port plant, a monopoly on the Ukrainian ammonia market, was held live on television. The minimum price for the asset was set at 4 billion hryvnias ($500 million). An obscure company called Nortima was declared the winner of the auction after bidding 5 billion hryvnias.

The other two bidders were Russia's Azot-Servis and Ukraine's Frunze-Flora.

However, just minutes after the winner was declared, the Ukrainian State Property Fund invalidated the offer, saying the price was too low, while Ukrainian Prime Minister Yulia Tymoshenko claimed a bidders' conspiracy.

"It became evident on live television how the three participants in the auction conspired to buy the Odessa Port plant for a song," Tymoshenko said.

The premier said that about 20 companies, including from Europe, had earlier expressed their intention to participate in the tender. However, litigations and a decree by Ukrainian President Viktor Yushchenko suspending the sale of the Odessa Port plant frightened off many potential buyers, she said.

The auction was intended to help the Tymoshenko government to replenish the state budget as Ukraine experiences a severe economic crisis. Yushchenko and Tymoshenko are set to run against each other in presidential elections in Ukraine in the New Year.