President Vladimir Putin has insisted that Russia's interests and security are non-negotiable, amid reports of more Russian troops moving closer towards Ukraine's borders.
Mr Putin gave a video address, hours after US President Joe Biden warned of "the beginning of a Russian invasion".
Russia was always "open for direct and honest dialogue", Mr Putin said, but he had full confidence in the military.
The West has announced a range of sanctions on Russian interests.
"We've cut off Russia's government from Western financing," Mr Biden said, after Russia's upper house of parliament authorised the president to send troops into two parts of eastern Ukraine controlled by Russian-backed separatists.
Mr Putin declared on Monday night that Russia had recognised the independence of the so-called people's republics of Luhansk and Donetsk, tearing up a peace accord with Ukraine.
Ukraine's foreign ministry urged all its citizens to leave Russia, warning that the "escalating Russia aggression against Ukraine" could limit consular assistance. The military in Kyiv also announced it was immediately calling up all reservists aged 18 to 60, both officers and privates for a maximum of a year.
Meanwhile, in the rebel-held areas, separatist leader Denis Pushilin said military mobilisation was gathering pace to counter what he described as Ukrainian aggression, adding that he could also ask Russia for help. Appearing alongside him, a senior official from Russia's ruling United Russia party said 93,000 people had been evacuated to Russia.
Moscow has also begun evacuating its embassy in Kyiv and has lowered its flag there, reports say.
This video can not be played
President Putin's claim that the military would go to "maintain peace" was derided as nonsense by the West. Rejecting his spurious claim of genocide in eastern Ukraine, UN Secretary-General António Guterres said he was concerned about "the perversion of the concept of peacekeeping".
Mr Biden said "to put it simply, Russia just announced that it is carving out a big chunk of Ukraine".
Despite Mr Putin's insistence that he was still open to diplomacy, France's foreign minister and US Secretary of State Anthony Blinken both cancelled planned meetings with Russia's foreign minister, Sergei Lavrov.
French Foreign Minister Yves Le Drian later said Mr Putin's aim was to "negate" Ukraine as a sovereign country. German Foreign Minister Annalena Baerbock said Mr Putin was trying to turn back the wheel of history. But both said they were open to further negotiations with Moscow.
It is not yet clear if any Russian troops have yet crossed the border into Ukraine. However, US satellite imagery has highlighted several new troop and equipment deployments in western Russia, and more than 100 vehicles at an airfield in Belarus near Ukraine's border.
Mr Biden said he was announcing an initial tranche of measures, targeting high-ranking Russia's "elites" and its foreign debt, to prevent it raising state financing from Western financial institutions.
The European Union and UK also announced a wave of sanctions against Russian banks and individuals on Tuesday:
European Commission Vice President Vera Jourova said it was a mistake to exclude the Russian leader. "Vladimir Putin should be first on the list," she told Czech daily Hospodarske Noviny.
British Foreign Secretary Liz Truss said the UK would use "every lever at our disposal" to stop Russia's threats against Ukraine. Responding to criticism from some MPs that the UK government had not gone far enough Ms Truss wrote in The Times that G7 allies had agreed on further sanctions and "nothing is off the table".
Tap or click to go through the gallery
Ukraine's President Volodymyr Zelensky has said he will consider cutting off diplomatic ties with Russia altogether over the crisis.
Russian-backed rebels in eastern Ukraine have seized large swathes of territory since fierce fighting broke out in 2014. An estimated 14,000 people – including many civilians – have since died.
Are you in Ukraine? Are you or your family affected? Email firstname.lastname@example.org.
Please include a contact number if you are willing to speak to a BBC journalist. You can also get in touch in the following ways:
If you are reading this page and can't see the form you will need to visit the mobile version of the BBC website to submit your question or comment or you can email us at HaveYourSay@bbc.co.uk. Please include your name, age and location with any submission.
Germany halts pipeline as nations sanction Russia
The Nord Stream 2 pipeline and the Ukraine crisis
What sanctions are being imposed on Russia?
PM announces UK sanctions against Russia
Gun battles as Russian troops reach Mariupol city centre, says mayor
Young boys found in Amazon after four weeks
Nasa's giant new Moon rocket makes its debut
In maps: Russia 'tightening noose' around Mariupol
On the front line as Ukraine's army stands firm
Putin lays out his demands in Turkish phone call
Elegant Odesa is transformed by efforts to deter Russians
Arnie's anti-war film trends on Russian social media
Is the world ready for a great reopening?
Quiz of the week: Who made headlines at the Baftas?
Elizabeth Holmes' former partner goes on trial
Tiny town opening its arms to Ukrainian refugees. Video
'I was scrolling for 13 hours a day'
How has Instagram become the dominant force it is today?
Has social media affected porn?
Louis Theroux explores the issue with consent in the industry
Was this just a freak accident?
How one ship triggered a global crisis…
© 2022 BBC. The BBC is not responsible for the content of external sites. Read about our approach to external linking.