KIEV, July 5 - Ukraine's SBU security service said on Tuesday it has opened a criminal investigation into the activity of an energy company once run by former Prime Minister Yulia Tymoshenko.

Tymoshenko, an opposition leader, is already on trial charged with abuse of office over a 2009 gas deal with Russia and is the target of two other criminal cases relating to her activities as prime minister.

She has dismissed all charges as politically motivated and accused the government of President Viktor Yanukovych, who narrowly beat her in the 2010 presidential election, of cracking down on opposition.

SBU said in a statement that United Energy Systems of Ukraine, a company that once imported Russian gas for resale in Ukraine, along with some former government officials, had tried to steal $405 million from the state budget.

The security service said it opened the probe after Russia had asked Ukraine to pay off that company's debt. Tymoshenko denies the debt exists.

Tymoshenko ran United Energy Systems in 1995-96, earning herself the nickname "gas princess."

She already faces up to 10 years' prison if found guilty on abuse of power charges and her case has raised concerns in the West, clouding Ukraine's plans to forge an association deal with the European Union.

In the current trial, the prosecution alleges that Tymoshenko, who was twice prime minister, forced the then-head of state-owned Naftogaz to sign the 2009 deal with Russia's Gazprom without consulting her government. Tymoshenko denies this.

Yanukovych's administration says the agreement was a sell-out of national interests, though it is abiding by the terms.

Separate cases are pending against Tymoshenko over alleged misuse of funds received in exchange for carbon emission quotas and over purchases of emergency rescue cars by her government.

Since Yanukovych came to power, several former members of Tymoshenko's cabinet have been prosecuted for alleged offences in office and at least one has fled Ukraine.

Western governments have not publicly supported Tymoshenko but have expressed concerns over the possible use of "selective justice" in Ukraine and "the appearance of a political motive" in her case.

Yanukovych has repeatedly denied that politics was involved and said his government was merely fighting corruption. (rt/ez)