Ukrainian President Volodymyr Zelensky has praised Western backing in the face of Russia’s military build-up and announced an expansion of the army.
“Support for Ukraine is the biggest since 2014,” he told MPs on a day of visits by British, Polish and Dutch leaders.
Top US and Russian officials are due to hold fresh talks on the crisis.
Mr Zelensky told parliament a new “format of political co-operation” was being created with the UK and Poland.
Ukrainian Foreign Minister Dmytro Kuleba said it would cover security as well as trade, investment and energy.
Speaking in Kyiv, Polish Prime Minister Mateusz Morawiecki promised defensive weaponry including drones and anti-aircraft missiles but also humanitarian aid. Poland has said it is preparing for a potential big influx of Ukrainian refugees if Russia attacks.
Russia has repeatedly denied planning any invasion of Ukraine, but has deployed an estimated 100,000 troops as well as tanks, artillery and missiles within reach of its borders.
UK Prime Minister Boris Johnson arrived in Kyiv on Tuesday, with a promise of £88m ($118m) to promote stable governance and energy independence from Russia.
While Poland and the UK are part of Nato’s mutual defensive alliance, Ukraine is not, and no Nato member has offered to send troops in the event of a Russian military attack.
Mr Zelensky told Ukraine’s parliament, the Rada, that he had signed a decree to increase the size of the Ukrainian army by 100,000 active soldiers, with the formation of 20 new brigades over three years while phasing out compulsory military service. Ukraine’s professional army is vastly outnumbered by Russia’s.
“The decree is not because a war is coming soon. I am saying this to everyone. The decree is for peace in Ukraine soon and onwards,” he stressed.
European leaders were also talking to Russia’s Vladimir Putin. Italian Prime Minister Mario Draghi stressed the need for de-escalation during a phone conversation, his officials said. Meanwhile, Hungary’s Viktor Orban assured the Russian leader in person that no EU leader wanted war.
However, there were angry clashes between Russian and US envoys during a meeting of the UN Security Council late on Monday.
US Ambassador Linda Thomas-Greenfield described Russia’s mobilisation as the biggest Europe had seen in decades. “And as we speak, Russia is sending even more forces and arms to join them,” she said, accusing Russia of planning to increase its force on Ukraine’s northern border in Belarus to 30,000.
Her Russian counterpart, Vasily Nebenzya, accused the US of “whipping up tensions and rhetoric, and provoking escalation” as well as of unacceptable interference in Russia’s internal affairs.
Russia often deployed troops on its own territory and that this was none of Washington’s business, he added.
Despite the angry rhetoric, US Secretary of State Antony Blinken is due to speak by phone with Russian Foreign Minister Sergei Lavrov on Tuesday.
The US said it had received a written response from Russia to a US proposal aimed at de-escalating the crisis in Ukraine. But hours later Russia’s deputy foreign minister said that was not true and a source told Ria news agency it was still preparing a response.
Late on Monday, the US ordered the departure of family members of American government employees from Belarus, citing the “unusual and concerning Russian military build-up”. A similar order was earlier issued to families of US government personnel in the American embassy in Ukraine’s capital Kyiv.
Russia has demanded a series of “security guarantees” from the West, including a promise that Ukraine will never be allowed to join the 30-member Nato alliance.
That demand has been rejected, along with Russia’s insistence that Nato allies stop deploying troops and military equipment in Eastern Europe.
In reality, President Vladimir Putin is seeking to roll Nato back to pre-1997 borders, so no combat units are deployed in Poland or the Baltics, all former members of Russia’s communist-era Warsaw Pact defence alliance.
Russia annexed Ukraine’s southern Crimea peninsula in 2014. It is also backing rebels who seized large swathes of the eastern Donbas region soon afterwards, and more than 14,000 people have been killed in the conflict.