KYIV, Dec 6 (Reuters) - Ukraine's President Volodymyr Zelenskiy on Monday said his armed forces were capable of fighting off any Russian attack, as the country marked its national army day with a display of U.S. armoured vehicles and patrol boats.

U.S. President Joe Biden has pledged his "unwavering support" to Ukraine in its standoff with Moscow and will hold talks with Russian President Vladimir Putin on Tuesday to try to defuse the crisis. Zelenskiy spoke to U.S. Secretary of State Antony Blinken on Monday.

Ukraine has accused Russia of massing tens of thousands of troops near its border in preparation for a possible large-scale military offensive, raising the prospect of open war between the two neighbours.

"The servicemen of the Armed Forces of Ukraine continue to fulfil their most important mission - to defend the freedom and sovereignty of the state from the Russian aggressor," Zelenskiy said in a statement.

"The Ukrainian army ... is confident in its strength and able to thwart any conquest plans of the enemy," he said.

Russia has dismissed talk of a new assault on Ukraine as false and inflammatory but told the West not to cross its "red lines" and to halt the eastward expansion of the NATO alliance.