Russia today warned that Ukraine was on the brink of "civil war" after Kiev's leaders pushed troops and tanks toward a flashpoint eastern city to counter a separatist surge backed by Moscow.
The 20 tanks and armoured personnel carriers were the most forceful response yet by Kiev's Western-backed government to the pro-Kremlin militants' occupation of state buildings in nearly 10 cities across Ukraine's rust belt.
Ukrainian forces set up a cement road barrier and began checking traffic leading to Slavyansk while fighters and attack helicopters circled overhead.
The economically depressed industrial city of 100,000 that has been under effective control of separatist gunmen since Saturday.
"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.
He insisted that the militants were receiving support from several hundred soldiers from the Russian army's Main Intelligence Directorate (GRU) that had been dispatched to Slavyansk and surrounding villages.
Witnesses told AFP that at least two Ukrainian military helicopters had landed in the nearby town of Kramatorsk with reinforcements for the offensive.
One Slavyansk resident told Kiev's private ICTV television that the insurgents had begun reinforcing barricades and set up two machineguns along one of the main entrance to the city.
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".
He urged Ukraine's "de-facto" authorities -- not recognised by Moscow -- to avert "terrible turmoil".
Russian President Vladimir Putin called for the United Nations to condemn Kiev's actions in east Ukraine, the Kremlin said.
But the White House said that Ukraine faced an "untenable" situation and described its military operation against pro-Russia militants as "measured".
White House spokesman Jay Carney urged restraint and "all due caution" while adding that the Ukrainian government had a "responsibility" to safeguard law and order.
