The unrest started in the east and south of the former Soviet republic after pro-European protesters forced the Kremlin-backed government in Kiev from power on February 22.
Below is a timeline of the main developments in the armed uprising in Ukraine.
6: Pro-Russian demonstrators seize government buildings in several towns in eastern Ukraine, including Donetsk, Kharkiv and Lugansk.
7: Pro-Russians occupying offices in Donetsk declare an "independent republic". Ukraine's government accuses Russia of wanting to invade the country and break it up.
12: Pro-Russian militants launch offensives in the towns of Slavyansk and Kramatorsk, where they take over the police headquarters.
13: Kiev announces an "anti-terrorist" operation in the east.
16: Ukrainian troops turn back from Slavyansk, after pro-Russians seize six armoured vehicles. Militants seize Donetsk town hall.
17: A deal is reached in Geneva between Ukraine, Russia, the United States and the European Union to "de-escalate" tensions.
18: The Kremlin confirms Russia has built up troops on the border. NATO puts the deployment at 40,000.
20: The leader of pro-Russian demonstrators in Slavyansk appeals to Moscow to send in peacekeeping troops after a deadly shootout.
21: In Lugansk, protesters pledge to hold their independence referendum on May 11.
22: In Kiev, US Vice President Joe Biden says Russia faces "isolation". Washington orders 600 soldiers to Poland and the ex-Soviet Baltic states.
23: Russia says it will respond if its interests are attacked in Ukraine.
24: Ukraine's military launches assault on Slavyansk. Five rebels are killed, according to Kiev. Russian President Vladimir Putin says there will be "consequences".
25: Ukrainian Prime Minister Arseniy Yatsenyuk accuses Moscow of seeking to trigger a "third world war". A group of OSCE military observers is detained in Slavyansk.
26: Yatsenyuk cuts short a Vatican trip, saying Russian warplanes violated Ukraine's airspace seven times. Russia denies the claim.
27: Rebels say the OSCE observers are "prisoners of war" and make them give a news conference.
28: West imposes new sanctions on Moscow. Russia vows a "painful" response. NATO says there is no sign of a Russia troop pullback claimed by Moscow. Russia assures the US it will not invade Ukraine.
29: Russia warns sanctions could harm Western interests in Russia's lucrative energy sector, denies there are Russian forces in eastern Ukraine.
30: Kiev places its armed forces on "combat alert" against a possible Russian invasion. Authorities admit local security forces are "helpless" to stop rebels over-running eastern towns. The IMF approves $17-billion aid deal for the beleaguered Ukrainian economy.
1: Some 300 pro-Russian militants seize the prosecutor's office in Donetsk amid violent clashes with police. Conscription reintroduced for all Ukrainian men aged 18-25.
2: The bloodiest day since new government came to power. At least 10 die in fresh army assault on Slavyansk. In the southern city of Odessa, 42 die when clashes between pro-Russian militants and pro-Ukraine supporters culminates in a massive blaze. Russia declares the Geneva peace plan dead and calls an emergency UN Security Council meeting. Obama says will step up sanctions if Moscow's actions prevent a planned May 25 presidential election in Ukraine.
3: Slavyansk rebels free OSCE inspectors after Kremlin envoy intervenes. Two days of mourning proclaimed over Odessa deaths. Fierce firefights around Slavyansk and nearby towns as military seeks to retake control.
4: Kiev vows to expand "anti-terrorism" operations. Yatsenyuk launches probe into security forces in Odessa over the deaths.