The Ukrainian ambassador to the United Nations urged other member nations to "stop the war," at an emergency meeting of the UN Security Council.

"It is the responsibility of this body to stop the war. So I call on every one of you to do everything possible to stop the war," Sergiy Kyslytsya said to the council, before turning to face the Russian ambassador to the UN and asking: "Or should I play the video with your president declaring the war?"

The Russian ambassador responded, "This isn't called a war, this is called a special military operation in Donbas."

Early Thursday morning, Russian President Vladimir Putin announced in a televised address that troops were taking military action in the Donbas region of eastern Ukraine.

CNN reporters and witnesses in cities across Ukraine have reported hearing explosions in the early hours of Thursday.