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.
Ukraine: people on the streets
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Open Image ModalA Maidan self-defence activist, holding a club and an axe, outside the Ukrainian Parliament in Kiev on 15 April 2014, during a rally of protesters who demand authorities to take actions against separatism on the eastern part of the country. (credit:Getty Images)
(02 of12)
Open Image ModalMaidan self-defence activists stop protesters trying to go inside the Ukrainian paliament in Kiev during a session on 15 April 2014. Russian Foreign Minister Sergei Lavrov on Tuesday warned Kiev against using force to quell pro-Moscow separatists in eastern Ukraine, saying the 'criminal' act would undermine talks planned in Geneva. (credit:Getty Images)
(03 of12)
Open Image ModalMaidan self-defence activists guard the Ukrainian Parliament in Kiev on 15 April 2014 as protesters outside the building demand authorities to take actions against separatism on the eastern part of the country. (credit:Getty Images)
(04 of12)
Open Image ModalPro-Russian activists guard a barricade outside the regional police building they seized in the eastern Ukrainian city of Slavyansk on 15 April 2014. Ukraine's acting president accused Russia of harbouring 'brutal plans' to destabilise the southeast of his ex-Soviet country by backing separatist militants. (credit:Getty Images)
(05 of12)
Open Image ModalPro-Russian activists guard a barricade outside the regional police building they seized in the eastern Ukrainian city of Slavyansk on 15 April 2014. (credit:Getty Images)
(06 of12)
Open Image ModalA man holding a rosary stands with Maidan self-defence activists who guard the Ukrainian Parliament in Kiev on 15 April 2014, as protesters outside the building demand authorities to take actions against separatism on the eastern part of the country. (credit:Getty Images)
(07 of12)
Open Image ModalAn elderly protester cries as she takes part in a rally outside the Ukrainian Parliament guarded by Maidan self-defence activists in Kiev on 15 April 2014. (credit:Getty Images)
(08 of12)
Open Image ModalPro-Russian activists guard a barricade outside the regional police building they seized in the eastern Ukrainian city of Slavyansk on 15 April 2014. (credit:Getty Images)
(09 of12)
Open Image ModalA pro-Russia activist guards a barricade outside the regional police building in the eastern Ukrainian city of Slavyansk on 15 April 2014. (credit:Getty Images)
(10 of12)
Open Image ModalA protester holds an icon as pro-Russian activists attend a rally outside the regional administration headquarters in the eastern Ukrainian city of Lugansk on 14 April 2014. Ukraine's interim president made a dramatic about-face aimed at defusing tensions in the separatist east by backing a national referendum on turning the ex-Soviet republic into a federation with broader regional rights. (credit:Getty Images)
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Open Image ModalA priest holds a cross and an Orthodox icon and prays as Pro-Russian activists attend a rally outside the regional administration headquarters in the eastern Ukrainian city of Lugansk on 14 April 2014. (credit:Getty Images)
(12 of12)
Open Image ModalProtesters hold placards depicting acting Ukrainian Interior Minister Arsen Avakov with sign reading 'Avakov - whose side are you on? Either stop separatists or get out!' during their protest action at the ministry in Kiev on 14 April 2014. Protesters demand to a stop to the inactivity of the Interior ministry and the Security Service in the eastern part of Ukraine. (credit:Getty Images)