The party of former Ukrainian premier Yulia...

The deal, signed by the Russian and Ukrainian presidents on April 21, extends the lease on the Russian base in the port of Sevastopol for at least 25 years after the current lease was to expire in 2017.

Ukraine signed the agreement, which the opposition has called an "act of treason," in exchange for securing a 30% discount for the Russian natural gas supplies.

"I do not exclude... the use of force," Yulia Tymoshenko Bloc lawmaker Olegh Lyashko said at a news conference, as quoted by UNIAN.

"They [the Ukrainian authorities] are determined in their decision to trade off Ukrainian national interests, and we are determined to defend them," Lyashko said.

He also said that the ratification of the deal could lead to a referendum on Crimea, with its majority Russian-speaking population, breaking away from Ukraine and joining Russia. However, Moscow has never made any claim to the peninsula.

Tymoshenko has called the deal with Russia a "shocking trade-off"

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