Ukraine`s main opposition leader, on the eve of a trip to the U.S., warned Saturday that the former Soviet republic is at risk of sliding back under the influence of Russia, according to AP.

Yulia Tymoshenko said she will reassure U.S. leaders on her visit starting Sunday that the Orange Revolution team which set Ukraine on its pro-Western path has reunited and will provide tough opposition to Prime Minister Viktor Yanukovych`s Russian-leaning government.

"Our union today is not due to circumstances, it is not a spontaneous decision," Tymoshenko told The Associated Press after signing an agreement Saturday to rejoin forces with President Viktor Yushchenko`s party.

"It is a decision dictated by those Ukrainians who want to see Ukraine European."

Tymoshenko was one of the driving forces behind the 2004 Orange Revolution, which helped bring the pro-Western Yushchenko to power. The Kremlin had backed Yanukovych, and his defeat was a major blow to Moscow`s efforts to keep Ukraine under its sway.

But Yushchenko`s hesitant governing style proved to be a disappointment for many Ukrainians who expected quick change and a strong embrace from Europe. He also split with Tymoshenko, a widely popular politician in Ukraine. Last year, Yanukovych`s party triumphed in parliamentary elections and he returned to power as prime minister, governing jointly with Yushchenko.

Yushchenko has since become sidelined, and Tymoshenko said that under Yanukovych, Russia`s influence was growing.

"I don`t want to be silent about this," she said, noting that Moscow`s pressure was particularly strong in the energy sector.

"Really, there is energy pressure on Ukraine which ... is used today for political control of the country," she said. "All this forces us to confront a new challenge: to protect the independence of our country."

Russia temporarily cut off natural gas supplies to Ukraine last year, a shut-off was widely seen as punishment for Ukraine`s pro-Western policies. This year, both sides agreed to a price widely seen as a gift to Yanukovych`s government — nearly half the price Russia is demanding from Georgia, another West-leaning ex-Soviet republic.

Russia has increasingly used its huge energy supplies to wield influence in other former Soviet republics and Eastern Europe. Last month, Russia stopped pumping oil to Belarus in a trade dispute. The stoppage affected Poland, Germany, Ukraine, Slovakia, the Czech Republic and Hungary — all of which are dependent on Russian oil.

Russia is also trying to exert pressure on Poland and the Czech Republic to turn down requests by the Washington to host parts of a U.S. missile defense system. Both countries have expressed interest in the system, which Moscow says could disturb the balance of power in the region and fuel a new arms race.

Tymoshenko`s visit to the U.S. comes after Russian President Vladimir Putin gave a speech in Germany earlier this month blasting U.S. foreign policy — his harshest criticism of the Bush administration since he came to power in 2000.

Tymoshenko is scheduled to meet with Vice President Dick Cheney and Secretary of State Condoleezza Rice in Washington.