Russian President Vladimir Putin ordered military forces into two Moscow-backed separatist areas of eastern Ukraine after recognizing their independence on Monday.
The move marked an escalation in the months-long stand-off and drew outrage from Western powers.
The Kremlin leader signed decrees to recognize the sovereignty of Luhansk and Donetsk, known as the Donbass, at a televised event on Monday evening in which he delivered a fiery speech questioning Ukraine's right to statehood.
It was not immediately clear when the soldiers would be sent in or in what numbers. The Kremlin described them as peacekeeping troops.
Putin also instructed Moscow's Foreign Ministry to establish formal diplomatic relations with the two areas held by pro-Russia rebels, which are part of the Donbass region and belong to Ukraine under international law.
Ukrainian President Volodymyr Zelensky said Kiev now expects "clear and effective steps of support from our partners" and that Moscow's recognition of Luhansk and Donetsk might constitute a unilateral withdrawal from the Minsk Protocol, an agreement struck between Russia and Ukraine in 2015 to resolve the conflict in the Donbass region.
Zelensky said Kiev would not react to provocations but would also not give up any territory. He seemed to be seeking to ease fears with the public address, saying there was no reason for sleepless nights.
At an emergency UN Security Council meeting in New York, Kiev's ambassador to the UN called on Russia to reverse its recognition of the two territories and return to negotiations, while Russia's representative accused Kiev of planning to shell the regions and threatened "extremely dangerous consequences" to "militaristic plans" from Ukraine.
Emergency meeting
Meanwhile, there was a chorus of condemnation to Putin's move from world leaders, with many calling it a sign that he was no longer interested in discussions over the crisis.
US envoy to the UN Linda Thomas-Greenfield said at the emergency meeting that Putin's claim he was sending in forces as peacekeepers was "nonsense" and said Moscow's latest move was "clearly the basis for Russia's attempt to create a pretext for a further invasion of Ukraine."
UN Secretary General António Guterres had earlier said Russia's recognition of the breakaway republics violates the UN Charter.
The European Union and Britain said they were preparing sanctions against Russia.
In Washington, President Joe Biden issued an executive order prohibiting investment, trade and financing by US individuals in the separatist region, with further measures to be announced Tuesday.
The US also moved diplomatic staff out of Ukraine entirely and said they would stay in Poland on Monday night amid the escalation in the crisis.
There have been worries in Washington and European capitals for weeks that Moscow could use the recognition of independence in the Donbass region as a way to move in troops.
South Ossetia, Abkhazia
Russia did something similar in 2008, when the regions of South Ossetia and Abkhazia broke away from Georgia. After a war in Georgia, Russia recognized the regions as independent states in 2008. That allowed Moscow to station thousands of soldiers there.
Fighting has flared in recent days in the Donbass between the Russian-backed rebels and Ukrainian forces, with fears that it could become the flashpoint for a wider war and Russian invasion.
Moscow has staunchly denied for weeks having any such plans, despite amassing some 150,000 troops on Ukraine's border.
The pro-Russian separatist leaders in the two regions had earlier asked Putin for assistance in the fight against Ukrainian government troops, and last week the Russian parliament urged Putin to recognize regions.
Putin held a televised meeting on Monday afternoon of his Security Council in which all of the participants, including Foreign Minister Sergei Lavrov and Defence Minister Sergei Shoigu, spoke in favour of recognition.
Hours later, Putin went on television to announce the decision and deliver a fierce denunciation of the Western-backed Ukrainian government in Kiev.
Putin: Ukraine a US 'colony'
Ukraine never had "real statehood," is a "colony" of the US and led by a "puppet" regime, he said.
As part of its solution to defusing tensions, Russia has been seeking a guarantee from NATO that Ukraine will never become a member of the alliance, a demand that has been repeatedly rebuffed.
In the hours preceding Putin's announcement there had been more reports of violence along the frontlines in eastern Ukraine.
NATO has warned that Moscow could use an alleged Ukrainian attack on Russia as a pretext for an invasion.
More than 14,000 people have died in periodic fighting since the conflict erupted in 2014 following Russia's annexation of the Ukrainian peninsula of Crimea, with a ceasefire and peace plan put in place in 2015 largely ignored.
The Organization for Security and Co-operation in Europe said its war monitors had logged more than 3,000 ceasefire violations between February 18 and 20.
