What sanctions have been imposed on Russia? Europe Editor James Mates explains
Ukrainian President Volodymyr Zelenskyy has said he is activating some reservists, but not ordering full mobilisation, as the threat of a Russian invasion grows.
It comes after Russia's parliament granted President Vladimir Putin permission to use force outside of the country, further fuelling fears of a broader attack on Ukraine.
On Monday night, Mr Putin ordered "peacekeeping" troops into Ukraine's Donetsk and Luhansk regions, just hours after he recognised them as independent.
James Mates explains what can be expected in the days and weeks ahead
Speaking on Tuesday night, US President Joe Biden slammed Russia's actions as a "flagrant violation of international law", warning "this is the beginning of a Russian invasion of Ukraine".
The US leader added that "the first tranche" of sanctions will be imposed on a number of Russian banks, the nation's sovereign debt and a group of extremely wealthy Russian individuals.
The US is just the latest nation to announce sanctions on Tuesday.
Following an early morning Cobra meeting, Boris Johnson revealed the “first barrage of UK economic sanctions against Russia,” hitting five Russian banks and three "very high net wealth" individuals.
In the European Union, member states also agreed to adopt a package of sanctions against Russia.
The EU’s foreign affairs policy chief Josep Borrell said it was a “particularly dangerous moment for Europe” and Russia’s actions were a “clear escalation of the Russian aggression against Ukraine”.
Mr Borrell said that 351 members of the Russian State Duma who “voted (for) this violation of international law” would be sanctioned, as well as “27 individuals and entities who are playing a role in undermining or threatening Ukrainian territorial integrity, sovereignty, and independence”.
These included “banks that are financing Russia” and “those who waged this information war against Ukraine”. He added the bloc would also “target the ability of the Russian state and government to access to our capital and financial markets and services”.
Mr Borrell added the package would “hurt Russia, and it will hurt a lot”.
Global Security Editor Rohit Kachroo on whether Russia is on the cusp of a much larger invasion
The toughest penalty on Russia so far came from Germany, which has halted approval of a major gas pipeline from Russia amid fears an invasion of Ukraine was just "hours away".
The Nord Stream 2 gas pipeline bringing natural gas from Russia to Germany has long been criticised by the United States and some European countries who argue that it increases Europe’s reliance on Russian energy supplies.
Alongside sanctions, western nations are using troops in a bid to dissuade Russia from a full-scale invasion.
ITV News US Correspondent Emma Murphy explains how much difference President Biden's response to the latest Russian movements will make
At Tuesday's press conference, President Biden said he was moving US personnel to the Baltic states on Nato’s eastern flank bordering Russia. The prime minster of Estonia and presidents of Latvia and Lithuania had on Friday made a direct plea for the US to step up its presence in the Baltics.
Meanwhile, the Russian Foreign Ministry claim that its diplomats in Ukraine had received threats. The ministry said, for this reason, it will evacuate its diplomatic personnel "in the nearest time".
Russia's leader blamed the current crisis on Ukraine's ambitions to join Nato, calling for recognition of Crimea as part of Russia and a halt to weapons shipments to Ukraine.
ITV News Correspondent Neil Connery explains why understanding the past is crucial to making sense of the present
President Putin went further than he did the night before, when he recognised the separatist areas of Donetsk and Luhansk as independent, saying Moscow recognises the separatists' claim to the entire Donbas region (including the parts which are Ukraine-controlled).
The Russian leader added that the way forward is for Ukraine to pledge neutrality and abandon its aim of joining Nato.
Kremlin spokesperson Dmitry Peskov reasoned that Russia had recognised the breakaway regions' independence “in borders that existed when they proclaimed” their independence in 2014.
Those borders now include areas currently under control of Kyiv after Ukrainian forces later reclaimed control of large parts of both regions during a nearly eight-year conflict that has killed more than 14,000 people. It was these Kyiv-controlled areas which Mr Putin said Russia now recognises as independent.
The Russian Duma also ratified President Vladimir Putin’s order to recognise Donetsk and Luhansk regions as independent, with all votes in favour.
Neil Connery reports live from Moscow on Russian President Putin's state of mind
Amid these moves by Russia, the UK's Foreign Office (FCDO) urged remaining Britons to leave Ukraine "immediately".
British Ambassador to Ukraine, Melinda Simmons, said as Russian troop numbers continue to grow, British nationals "should leave now while commercial options are available".
"If the situation continues to escalate, it is likely that commercial routes out of Ukraine will be severely disrupted and road routes through Ukraine may be closed," the ambassador said in a Twitter video.
What is the latest on the crisis facing Ukraine?
In a move that was deemed highly significant and an indicator of Europe's intentions, Germany put on ice a major $10 billion gas pipeline from Russia, Nord Stream 2.
Boris Johnson warned Mr Putin appeared to be "bent on a full-scale invasion" of Ukraine, as he announced the UK had imposed the "first barrage" of economic sanctions against Russia, hitting five Russian banks and three "very high net wealth" individuals.
The US has now recognised Russian troop deployments in eastern Ukraine as an "invasion" after initially hesitating to use the term, with President Joe Biden also vowing to impose severe sanctions.
Speaking at a late-night meeting of the UN Security Council on Monday, the UK's ambassador to the UN, Dame Barbara Woodward, urged Russia to "step back". "The humanitarian impact will be terrible on civilians fleeing the fighting," she said.
Remaining Britons were urged to leave Ukraine immediately
In Russia, tanks were seen rolling in the direction of Donetsk following President Putin’s order to send Russian forces in on Monday night.
Al-Jazeera and La Figaro published video of Russian military convoys moving into the separatist areas of east Ukraine.
The Kremlin has said Mr Putin is ordering Russian forces to “maintain peace” in eastern Ukraine.
Ukraine's President Zelenskyy, meanwhile, sought to project calm, telling the country in an address overnight: “We are not afraid of anyone or anything. We don’t owe anyone anything. And we won’t give anything to anyone.”
Nato secretary general Jens Stoltenberg said Russia's actions "further undermines Ukraine’s sovereignty and territorial integrity, erodes efforts towards a resolution of the conflict, and violates the Minsk Agreements, to which Russia is a party.”