Ukrainian President Volodymyr Zelenskiy signed a decree on Tuesday to boost his armed forces by 100,000 troops over three years and raise soldiers' pay, but said this did not mean war with Russia was imminent.

Zelenskiy urged lawmakers to stay calm and avoid panic as he prepared to host the leaders of the Netherlands, Britain and Poland - all NATO members - as part of efforts to defuse tension with Russia and shore up international support for Kyiv.

Russia has massed more than 100,000 troops near Ukraine's borders, while denying plans to invade - an action that the United States and its allies have warned would trigger tough sanctions.

The West last week formally rejected Russian demands to bar Ukraine from ever joining NATO and pull out NATO forces from eastern Europe, while expressing willingness to talk about arms control and confidence-building measures.

Russia has not yet signalled its next move, and the Kremlin reiterated on Tuesday that President Vladimir Putin would respond "when he considers it necessary".

Putin said last week the United States and NATO had not addressed Moscow's main security demands but Russia was ready to keep talking. On Tuesday he was due to meet Hungarian Prime Minister Viktor Orban and hold a news conference at 1300 GMT.

"WE MUST BE UNITED"

Ukraine's Zelenskiy said he signed the decree on expanding the armed forces "not because we will soon have a war... but so that soon and in the future there will be peace in Ukraine".

Despite the Russian troop build-up, Zelenskiy has repeatedly pushed back against warnings by the United States and other NATO allies that Russia could attack Ukraine at any moment.

"We must be united in domestic politics. You can be in opposition to the government, but you can't be in opposition to Ukraine," Zelenskiy said.

"You can despise ... the government, the president, but you can't despise your own people, sow panic in order to reap political gains, keep people in a state of alarm."

There are currently nearly 250,000 people in Ukraine's armed forces, compared to Russia's overall strength of around 900,000.

NATO member states have rallied round Ukraine in recent weeks, with the United States, Britain and Poland among countries offering military aid and calling for tough sanctions on Moscow if Russia launches an attack.

"We urge Russia to step back and engage in dialogue to find a diplomatic resolution and avoid further bloodshed," British Prime Minister Boris Johnson said in remarks released ahead of his arrival. "As a friend and a democratic partner, the UK will continue to uphold Ukraine's sovereignty in the face of those who seek to destroy it."

Any sanctions on Moscow would build on those imposed on Russia after it annexed Crimea and backed separatists fighting government forces in eastern Ukraine in 2014, but Europe's dependence on Russian energy supplies weakens the West's hand.

Russian Foreign Minister Sergei Lavrov was due to speak to U.S. Secretary of State Antony Blinken on Tuesday for the first time since Washington formally replied to Russia's security proposals last week.

A State Department spokesperson said on Monday the United States had received a written follow-up from Russia on the matter.

A senior diplomatic source told Russian news agency RIA the letter contained questions from Lavrov, also sent to other NATO members, on how Moscow's counterparts understood the notion of "indivisibility of security".

Moscow contends that NATO's addition of 14 new members in eastern Europe since the Cold War poses a threat to Russia, and that NATO is violating an agreed international principle that countries should not strengthen their own security at the expense of others. (Reuters)

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