Below is a chronology of the main developments surrounding the armed separatist uprising in eastern Ukraine, after Russia's annexation in March of the Crimea peninsula.

The unrest started in the east and south of the former Soviet republic after pro-European protesters forced the overthrow of the Kremlin-backed government in Kiev on February 22.

APRIL 2014

- 6: Tensions soar when pro-Russian demonstrators seize local 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, to the north of Donetsk, where they take over the police headquarters.

- 13: Kiev announces an "anti-terrorist" operation in the east. In Mariupol, protesters hoist the flag of the "republic" of Donetsk.

- 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. The separatists reject the accord.

- 18: The Kremlin confirms that Russia has built up its military presence 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 own local referendum on autonomy 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.

Ukraine orders a military operation against pro-Kremlin separatists to resume.

- 23: Russia says it will respond if its interests are attacked in Ukraine.

- 24: Ukraine's military launches an assault on Slavyansk. Up to five rebels are killed, according to Kiev.

Special forces seize back control of the town hall in the southeastern port city of Mariupol.

Putin says that deployment of military in east Ukraine by the Kiev authorities is a crime that will "have consequences". The Russian army starts new exercises on the border with Ukraine.

- 25: Ukrainian Prime Minister Arseniy Yatsenyuk accuses Moscow of seeking to trigger a "third world war".

Insurgents blow up an army helicopter with a rocket-propelled grenade at an air base close to Kramatorsk.

A group of OSCE military observers are detained in Slavyansk.

- 26: The G7 releases statement vowing to "move swiftly to impose additional sanctions on Russia."

Rebels in Slavyansk accuse detained OSCE observers of being "NATO spies".

Russia offers to help secure the observers' release but blames Ukraine for failing to protect them.

Yatsenyuk cuts short a Vatican trip to see Pope Francis due to the escalating crisis, says Russian warplanes violated Ukraine's airspace seven times overnight. Russia denies the transgressions.

- 27: Rebels say the OSCE observers are "prisoners of war" and present them in front of the press, where they say they have not been mistreated.

The OSCE sends a second team to negotiate their release. One man, a Swede, is freed on medical grounds.

Pro-Kremlin militants armed with baseball bats seize the regional television station in Donetsk.

- 28: In eastern Ukraine, gunmen storm the town hall and police offices in Kostyantynivka.

The pro-Russian mayor of the town of Kharkiv is seriously wounded by gunfire.

The OSCE holds an emergency meeting on Ukraine.

Washington and the EU impose new sanctions on Moscow to which Russia vows a "painful" response.