Ukrainian President Petro Poroshenko has struck a defiant tone towards Russia, accusing his country's larger neighbour of vain attempts at invasion as Ukraine celebrated 25 years of independence.

"Looking back over more than two years of war, it is possible to confidently say that the enemy has not been able to bring Ukraine to its knees," Poroshenko said at a war memorial in Kiev on Wednesday.

"For this, 2,504 of our soldiers have died," he continued.

The celebration, which featured a military parade in Kiev, came amid escalating tensions with Russia as Moscow boosts its military presence near the countries' shared border.

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Ukraine was a part of the Soviet Union until 1991 and, as such, under Moscow's de facto control.

On August 24, 1991, Ukraine's parliament adopted its Declaration of Independence, establishing the republic as separate from the disintegrating Soviet Union.

Efforts in recent years to tilt Ukraine towards the West have prompted divisions in the country, which Ukraine and most of the West have blamed on Russia.

Ukraine has fought against pro-Russian separatists backed by Moscow since early 2014, when Kiev deposed its pro-Russian president amid mass protests calling for closer ties with the West.

More than 9,500 people have died and two million forced from their homes in the fighting in two major industrial regions in the east.

Kiev also lost its strategic Black Sea peninsula of Crimea when it was seized by Russian soldiers on Putin's orders and annexed in March 2014.