A military jet circles the skies above Kramatorsk, north of Donetsk in Ukraine.
Ukraine has launched an 'anti-terrorist operation' against pro-Russian gunmen in the town of Slavyansk.
On the brink ... pro-Russia activists in front a so-called Donetsk Republic flag outside the regional police building in Slavyansk. Picture: Genya Savilov Source: AFP
RUSSIAN President Vladimir Putin has told the German chancellor in a phone call that Ukraine is on the verge of civil war, and blamed Kiev for the situation.
But both Mr Putin and Angela Merkel "emphasised the importance" of the planned four-way talks on Ukraine tomorrow between top diplomats of Russia, the European Union, the US and Ukraine.
The Kremlin has described the actions of the Ukrainian army in the country's east as anti-constitutional.
Mr Putin also told Ms Merkel about the importance of stabilising the Ukrainian economy and ensuring that supplies of Russian gas to Europe are not interrupted.
In this photo taken on Monday, April 14, 2014, Ukrainian soldiers sit on top of military vehicles parked in a field near the eastern Ukrainian town of Slovyansk. Source: AP
Kiev's leaders ordered troops and tanks towards a flashpoint eastern city to push back a separatist surge supported by Moscow.
The 20 tanks and armoured personnel carriers were the most forceful response yet by Kiev's Western-backed government to armed raids by pro-Kremlin militants and the occupation of state buildings in nearly 10 cities across Ukraine's rust belt.
Pro-Russia protester wearing gas masks storm a regional police building as one prepares a petrol bomb in the eastern Ukrainian city of Horlivka. Source: AFP
Ukrainian forces set up a cement road barrier and begun checking traffic leading to Slavyansk, an economically depressed industrial city of 100,000 that has been under effective control of separatist gunmen since Saturday.
Turmoil ... pro-Russian activists guard a barricade in Slavyansk. Picture: Genya Savilov Source: AFP
"They must be warned that if they do not lay down their arms, they will be destroyed," Ukrainian Security Service (SBU) General Vasyl Krutov told a group of reporters tracking the sudden tank movements.
A Pro-Russian activist stands during the mass storming of a police station in the eastern Ukrainian town of Horlivka. Source: AP
The Ukrainian government has launched an operation to uproot pro-Russia armed separatists who have seized government buildings in several cities in eastern Ukraine. Source: AP
Witnesses told AFP that at least two Ukrainian military helicopters landed in the nearby town of Kramatorsk with reinforcements for the offensive.
A pro-Russian activist makes barricades in front of the gate of an airport in Kramatorsk, eastern Ukraine, Tuesday, April 15, 2014. Source: AP
Kiev's response to the eastern insurgency prompted Russian Prime Minister Dmitry Medvedev to warn that "Ukraine is on the brink of civil war - it's frightening".
Pro-Russian men throw stones during the mass storming of a police station in Horlivka, eastern Ukraine. Source: AP
He urged Ukraine's "de-facto" authorities - not recognised by Moscow - to avert "terrible turmoil".
Reinforcements .... Ukrainian army troops set up a position at an airport in Kramatorsk, eastern Ukraine. Picture: Sergei Grits Source: AP
The rapid turn of events on the ground were preceded by a telephone conversation late Monday between Russian President Vladimir Putin and US President Barack Obama on Ukraine that was described as "frank and direct".
People sing the Ukrainian national anthem during a pro-Ukraine rally in the eastern Ukrainian city of Lugansk on April 15, 2014. Source: AFP
The call, though, appeared to break no new ground.
The Kremlin chief continued to reject any links to Russian-speaking gunmen who have proclaimed the creation of their own independent republic, and who have called on Putin to send in the estimated 40,000 Russian troops now stationed along the border with Ukraine.
Debastilising ... the road leading to the eastern Ukrainian city of Slavyansk, where pro-Kremlin militias who have seized buildings. Picture: Anatoliy Stepanov Source: AFP
But the White House said Obama accused Moscow of supporting "armed pro-Russian separatists who threaten to undermine and destabilise the government of Ukraine".
A Ukrainian soldier carries a box with ammunition as Ukrainian army troops receive ammunition in a field on the outskirts of Izyum, Eastern Ukraine. Source: AP
The worst East-West confrontation since the Cold War was exacerbated on the weekend by a Russian warplane "buzzing" a US destroyer in the Black Sea, and a visit to Kiev by CIA chief John Brennan that was confirmed by the White House and slammed by Moscow.
A Ukrainian soldier holds a hand grenade as Ukrainian army troops receive ammunition in a field on the outskirts of Izyum, Eastern Ukraine. Source: AP
European foreign ministers meanwhile held back on unleashing punishing economic sanctions against Russia in hopes that EU-US mediated talks on Thursday in Geneva between Moscow and Kiev could help de-escalate the situation.
'Frightening' ... a Ukrainian air force Su-27 fighter jet in the sky over Slavyansk, 160 kilometres from the Russian border. Picture: Alexander Ermochenko Source: AP
SOUTHEAST 'ON FIRE'
Kiev's untested interim leaders have struggled to meet the high-stakes challenge presented by the coordinated series of raids that began in the industrial hubs of Donetsk and Lugansk and have since spread to nearby coal mining towns and villages.
A masked pro-Russian activist guards a barricade at the regional administration building that they had seized earlier in Donetsk, Ukraine. Source: AP
The breakaway move, backed by tough talk by Moscow, could potentially see their vast nation of 46 million break up along its historic Russian-Ukrainian cultural divide.
Self defense volunteers line up outside the Parliament in Kiev, Ukraine, Tuesday, April 15, 2014. Source: AP
Moscow last month annexed the largely Russified region of Crimea after deploying military forces there and backing a hasty local referendum calling for the Black Sea peninsula to be absorbed into the Russian Federation.
Discontent ... Ukrainian army troops receive ammunition on the outskirts of Izyum, Eastern Ukraine. Picture: Sergei Grits Source: AP
But a forceful military response by Kiev could prompt a devastating counterstrike by Russian troops who are waiting to act on Putin's vow to "protect" Russian-speakers in the neighbouring state.
People gather during a rally in the Independence Square in Kiev, Monday, April 14, 2014. Source: AP
Ukraine's acting President Oleksandr Turchynov told an agitated session of parliament that the country was facing an eastern enemy rather than domestic discontent.
"They want to set fire not only to the Donetsk region but to the entire south and east - from Kharkiv to the Odessa region," the acting president said.
Unrest ... Kiev's response to the eastern insurgency prompted Russian Prime Minister Dmitry Medvedev to warn that "Ukraine is on the brink of civil war - it's frightening". Picture: Genya Savilov Source: AFP Source: AFP
'RUSSIAN COLONEL' IN RAID
Kiev's news programmes have spent much of the past day replaying footage from an attack by militants on a police station in the town of Gorlivka in which men in gas masks pelted the building with Molotov cocktails before smashing its windows with rocks and bats.
One clip showed a green-uniformed man who identified himself as a Russian colonel telling the local police force that it now must obey his orders and must wear orange-and-black ribbons symbolising Kremlin pride.
A deadline set by the Ukrainian government for pro-Russian gunmen to leave government buildings in eastern Ukraine and surrender weapons passed without incident early Monday, with no immediate sign of any action to liberate any seized buildings. Source: AP
The Kremlin set nerves in Kiev further on edge on Monday by announcing that Putin had received "a lot" of requests from eastern Ukraine "to help, to intervene in some form". Some in Kiev saw that as a new effort to create a pretext for an invasion.