Ukraine conflict: Poroshenko says future on 'brittle ice'
President Petro Poroshenko has marked Ukraine's independence day, warning of the need to act carefully in the next year in the face of "Russian aggression".
Ukraine was walking on "brittle ice" and the smallest false step could be fatal, he said.
Violence in the two eastern regions of Luhansk and Donetsk has escalated.
Russia has denied helping rebels in the east, despite a large body of evidence indicating its involvement.
President Poroshenko told Ukrainians that the threat of full-scale invasion remained, asserting that some 50,000 Russian troops were massed on Ukraine's eastern borders with a further 9,000 inside the self-declared rebel republics themselves.
Russia has insisted that any of its servicemen in the region are there on a voluntary basis.
Donetsk rebel spokesman Eduard Basurin said Mr Poroshenko's figures were nonsense, apparently given to put pressure on European leaders ahead of a meeting in Berlin later between the Ukrainian leader, Germany's Chancellor Angela Merkel and President Francois Hollande of France.
Russia annexed the Ukrainian peninsula of Crimea last year weeks before the conflict began in Luhansk and Donetsk.
A ceasefire signed in February is at risk of unravelling because of the recent flare-up in fighting near the Ukrainian port city Mariupol and the rebel-held town of Horlivka.
Mr Basurin accused Ukraine of moving heavy weapons to the front line and of violating the ceasefire 21 times in one day.
Almost 7,000 people have died and more than 17,000 have been wounded since the fighting began, according to UN officials.
Mr Poroshenko said some 2,100 Ukrainian soldiers had been killed.