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.
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.
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".
Do not make expressions casting dout on election: EC
14 Apr, 2022CM Bhatta says may New Year 2079 BS inspire positive thinking
14 Apr, 2022Three new cases, 44 recoveries in 24 hours
14 Apr, 2022689 climbers of 84 teams so far acquire permits for climbing various peaks this spring season
14 Apr, 2022How the rising cost of living crisis is impacting Nepal
14 Apr, 2022US military confirms an interstellar meteor collided with Earth
14 Apr, 2022Valneva Covid vaccine approved for use in UK
14 Apr, 2022Chair Prachanda highlights need of unity among Maoist, Communist forces
14 Apr, 2022Ranbir Kapoor and Alia Bhatt: Bollywood toasts star couple on wedding
14 Apr, 2022President Bhandari confers decorations (Photo Feature)
14 Apr, 2022