Here is a chronology of key events in Ukraine as Russia and the West face off.
FEBRUARY 18: Violence erupts as 20,000 pro-European protesters rally in Kiev to demand president Viktor Yanukovych be stripped of key powers.
- Protesters fight riot police on Independence Square, which demonstrators have occupied for three months to protest Yanukovych's decision to reject an EU trade deal in favour of closer ties with Russia.
20: Police open fire. An official toll lists 82 people killed in three days, including around 15 police.
22: Parliament impeaches Yanukovych as he flees Kiev condemning a "coup". Presidential elections are set for May 25.
Opposition leader Yulia Tymoshenko, who was sentenced to a seven-year jail term in 2011 for abuse of power, is released.
23: Parliament elects Tymoshenko ally Oleksandr Turchynov as interim president.
26: Russia questions the legitimacy of Ukraine's new leadership, puts armed forces in the region on high alert.
- Pro-Russian demonstrators brawl with supporters of interim authorities in Simferopol, capital of the Russian-speaking peninsula of Crimea.
27: Pro-Russian gunmen seize parliament and government buildings in Simferopol, raising the Russian flag.
- Parliament approves Tymoshenko ally Arseniy Yatsenyuk as prime minister heading a pro-Western cabinet.
28: Authorities in Ukraine say they have regained control of two Crimean airports seized during what the interior minister charged was an "armed invasion" by Russian forces.
- Yanukovych insists he has not been overthrown as he resurfaces at a news conference in Russia.
MARCH 1: Russian parliament authorises President Vladimir Putin to send troops into Ukraine. Ukrainian army adopts a state of alert.
- A referendum to determine whether Crimea residents want greater autonomy is brought forward to March 30.
- US President Barack Obama tells Putin he has violated international law and warns of reprisals. Putin insists Russia has the right to "protect its interests and Russian-speaking populations".
2: Yatsenyuk accuses Russia of declaring war.
- US Secretary of State John Kerry tells Russia it could lose its place in the prestigious Group of Eight.
- US official says Russian forces are "in complete operational control of the Crimean peninsula."
- Ukraine's navy chief switches allegiance to the pro-Russian authorities of Crimea, a day after he is appointed.
3: Russian Foreign Minister Sergei Lavrov criticises threats of "sanctions and boycotts". The foreign ministry says Kerry's warnings are "unacceptable".
- Ukraine accuses Russia of pouring more troops into Crimea. All military bases in Crimea are surrounded, the regional defence ministry says.
- The United States suspends military cooperation with Russia, the Pentagon says.
- 4: Putin denies Russia has sent troops into Crimea, says force will only be used as a last resort, admits Yanukovych has no political future and slams the takeover of power in Kiev as an armed coup. Obama says that Putin is not "fooling anybody".
- Kerry at a press conference in Kiev condemns Russia's "act of aggression" and says Washington is "not seeking confrontation" in Ukraine.
- Russian forces fire warning shots on Ukrainian soldiers at a base near Sevastopol.
- Yatsenyuk says that his government had made the first "timid" contacts with Russian leaders.
- Russian energy giant Gazprom says it will end Ukraine's gas discount from April but proposes a loan of up to $3 billion (2.2 billion euros) to Kiev to cover its debt.
- The EU will help Kiev pay what it owes to Gazprom, a top official says, as part of an aid package reportedly worth more than one billion euros.