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By Natalia Zinets KIEV (Reuters) - Ukraine's prime minister has launched what promises to be a bitter election campaign that could divide pro-Western parties and complicate their efforts to fight pro-Russian rebels in the country's east. Prime Minister Arseny Yatseniuk, a key interlocutor of the West during months of turmoil, announced on Thursday that he would quit, saying parliament was betraying Ukraine's army and people by blocking reforms supported by Western backers. His move, following the exit of two parties from the ruling coalition, amounted to the start of a campaign for seats in a legislature still packed with former allies of pro-Russian President Viktor Yanukovich, ousted by protests in February. "History will not forgive us," Yatseniuk told parliament on Thursday, in what analysts said was the first campaign speech for the party led by Yulia Tymoshenko, a rival of President Petro Poroshenko, who was elected to replace Yanukovich in May. Pro-Western political forces in Ukraine have been bitterly divided almost continuously since the country won independence with the breakup of the Soviet Union in 1991.