Ukraine's acting President has ordered an "anti-terrorist operation" against pro-Russian separatists.
In a statement to parliament on Tuesday, Olexander Turchynov said it was being conducted "stage by stage, in a responsible... manner".
Later on Tuesday, gunfire was heard at an airbase which had been under militant control, government officials claimed.
And in a speech on Tuesday, foreign secretary Willam Hague warned Russia not to miscalculate the response from the rest of the world.
"In recent days Russia has deliberately pushed Ukraine to the brink, and created a still greater risk of violent confrontation. We call on Russia to stop these actions and to condemn the lawless acts in eastern Ukraine," Mr Hague told an audience at the Mansion House in the City of London.
"Russia must choose whether it is open to diplomacy and de-escalation, and if it decides otherwise, we must be ready for a different state of relations with Russia in the next 10 years than in the last 20.
"Ukraine can be a bridge between East and West and be able to have good relations with Russia. But that does not entitle Russia to send in its armed groups, thinly disguised, to spearhead the occupation of buildings in multiple Ukrainian cities, to try permanently to destabilise the country and dictate the terms of its constitution.
"My message to Moscow is that if anyone thinks they can do these things without serious long-term consequences they are making a grave miscalculation."
Meanwhile, Ukrainians on both sides of the argument have been back on the streets as these powerful images illustrate.