KIEV, October 19 - RAPSI. The Russian Defense Ministry has been suing the United Energy Systems of Ukraine, which was headed by former Ukrainian Prime Minister Yulia Tymoshenko in the mid-1990s, for $405 million in debt for several years already, Russian Defense Minister Anatoly Serdyukov said on Wednesday.

"We have been suing the company for several years so far, and we are determined to stand fast," Serdyukov said.

Ukrainian mass media published Serdyukov's letter in June, in which he said the United Energy Systems owed the ministry over $400 million under 1997 contracts for industrial goods supplies.

The company was then headed by Tymoshenko who later became the prime minister.

ymoshenko was sentenced to seven years in prison on October 11 for abuse of office in connection with the gas contracts signed in 2009 between Russian Gazprom and Ukrainian Naftogaz.

The Ukrainian Security Service opened on October 13 a new criminal case against Tymoshenko. The case concerns the $405 million debt of United Energy Systems of Ukraine to Russia's Defense Ministry. Tymoshenko headed the company from 1995-1997. Investigators say Tymoshenko colluded with former Prime Minister Pavel Lazarenko to embezzle budget funds.