Ukraine's government vowed yesterday to press ahead with a military offensive against separatists, despite a deadly attack on an army helicopter, amid increasing reports that volunteer fighters from Russia were involved in rebellions in the east.

President-elect Petro Poroshenko, who scored an overwhelm swore to punish those responsible for the shooting down on Thursday of the helicopter near Slaviansk, which killed 14 servicemen including a general. Acting Defence Minister Mykhilo Koval, repeating charges that Russia was carrying out "special operations" in the east of Ukraine, said Ukrainian forces would continue with military operations in border areas "until there is peace".

Ukrainian authorities have long alleged that the rebellions have been fomented by Moscow among the largely Russian-speaking population, hostile to Kiev's "euromaidan" revolution that overthrew Moscow-backed President Viktor Yanukovich in February.

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Reports by Ukrainian border authorities and journalists on the ground now appear to show increasing evidence of direct involvement by volunteer fighters from Russia in the rebellions that erupted two months ago in the wake of Russia's annexation of Crimea.

Ukrainian border guards said yesterday they had seized a cache of weapons including machine-guns and grenade-launchers in cars they stopped as they crossed from Russia.

l An official of the Ukrainian border guard service said yesterday bodies of slain Russian nationals were being allowed to return to Russia for humanitarian reasons. "We don't need them to fertilise the land of Ukraine," Serhiy Astakhov, an aide to the head of the border guard service, said in Kiev in reply to a journalist.