The US has hit Russia with wide-ranging sanctions over what President Joe Biden called "the beginning of a Russian invasion of Ukraine".
"We've cut off Russia's government from Western financing," Mr Biden said.
It came after Russian politicians authorised President Vladimir Putin to send troops into two rebel-held parts of eastern Ukraine.
Russia has recognised the regions as independent - a move Ukraine said attacks its sovereignty.
Western nations largely see the action as a pretext for a wider invasion. On Monday Mr Putin ordered soldiers into the breakaway Donetsk and Luhansk regions, to "keep the peace" - a description the US called "nonsense".
Satellite imagery over the past 24 hours has shown several new troop and equipment deployments in western Russia, and more than 100 vehicles at an airfield in Belarus near Ukraine's border, according to US space technology company Maxar.
Russia's military has not commented, and it is not clear if any Russian troops have yet been sent into eastern Ukraine.
Announcing his "first tranche" of measures directly targeting the Russian state, Mr Biden said on Tuesday: "To put it simply, Russia just announced that it is carving out a big chunk of Ukraine."
The moves include sanctions on Russia's foreign debt, meaning the country can no longer raise money for its state financing from Western financial institutions.
The US is also punishing high-ranking "elites" from Russia.
"They share the corrupt gains of the Kremlin policies, and should share in the pain as well," Mr Biden said.
The US had already banned American companies from dealing with the rebel-held Ukrainian regions of Donetsk and Luhansk - but few companies do.
The UK and European Union also announced a wave of sanctions against Russian banks and individuals earlier on Tuesday.
None of the sanctions went as far as what had been threatened in the event of a full-scale invasion of Ukraine, though both US and European officials stressed that more action would follow if the situation deteriorated.
The US is also moving troops into Nato member states Estonia, Latvia and Lithuania - which are on or near Russia's border - an official said.
The military redeployment from Italy involves some 800 infantry troops along with a number of F-35 fighter jets and 20 attack helicopters, some of which will go to Poland.
Diplomatic efforts to avert the Ukraine crisis have so far failed, and in Russia on Tuesday President Putin said the Minsk agreements - the peace deals aimed at averting years-long conflict in eastern Ukraine - "do not exist".
The agreements "were killed long before yesterday's recognition of the people's republics of Donbas," he said, using the administrative name for the two regions of eastern Ukraine. "And not by us, not by representatives of these republics, but by the current Kyiv authorities."
US Secretary of State Antony Blinken followed Mr Biden's speech by announcing he had cancelled a planned Thursday meeting with his Russian counterpart.
"Now that we see the invasion is beginning and Russia has made clear its wholesale rejection of diplomacy, it does not make sense to go forward with that meeting at this time," he said.
But he added that the US would engage with Russia if it changed its approach.
Ukraine's President Volodymyr Zelensky earlier said he would consider cutting off diplomatic ties with Russia altogether over the crisis.
In an evening address to the nation, he said he was calling up military reserve members for training - but also said Ukraine would still pursue a diplomatic solution.
There was "no need" for a full military mobilisation, Mr Zelensky said.
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.
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