Lavrov on Friday accused Washington of seeking to "seize" Ukraine, as Kiev said Moscow wants to start a third World War.

Russia Foreign Minister Sergei Lavrov on Friday accused Washington of seeking to "seize" Ukraine, as Kiev said Moscow wants to start a third World War.

Lavrov warned "the might of US propaganda" was aimed "at smearing Russia, smearing those who protest against the illegal actions of the (Kiev) authorities who are trying to ban Russian and trying to call all Russians and Russian speakers enemies who should be killed."

"If truth be told, the West -- that's how it all started -- wants to seize Ukraine so to speak, being solely motivated by its own geopolitical ambitions and not the interests of the Ukrainian people."

Earlier on Thursday, Kerry warned that Moscow's refusal to end the Ukraine crisis would be costly, saying the window of opportunity for Russia to change tack was closing.

For his part, Ukrainian Prime Minister Arseniy Yatsenyuk accused Russian of wanting to start "a third world war" over the current crisis in the country, as he called for international help against "Russian aggression".

"The aggressive efforts of the Russian military on Ukraine's soil will lead to a conflict on European soil," he said in a televised cabinet meeting.

"The world hasn't forgotten the Second World War and Russia wants to start a third world war," he said.

"Russia's support for the terrorists in Ukraine constitutes an international crime and we call on the international community to unite against the Russian aggression."

Earlier on Thursday Kiev’s Western-backed government went ahead with its offensive against separatists in the country’s east, killing five pro-Kremlin activists.

Also on Thursday, Russia announced it has launched new military exercises on the border with Ukraine in response to Kiev’s operation.