US Vice President Joe Biden began a whistle-stop tour to Poland and Lithuania , meeting leaders of both states, together with those of Estonia and Latvia.

All four countries claimed to be in fear of a military assault by Russia in light of Moscow's annexation of Crimea.

"This trial, this challenge that we are facing, will not be for a month or a year. We are facing a strategic perspective for many years to come," said Polish Prime Minister Donald Tusk.

Mr Biden said that more sanctions would be applied against Russia.

"Russia has offered a variety of arguments to justify what is nothing more than a land grab, including what he said today, but the world has seen through Russia's actions and has rejected the flawed logic behind those actions," he declared.

President Barack Obama did his bit to keep the pot bubbling by inviting leaders of the G7 group of nations to a meeting in Europe next week to discuss further action.

The group was expanded to G8, including Russia, but French Foreign Minister Laurent Fabius said that G8 leaders had resolved to suspend Russia's participation - although this was news to member countries Japan and Germany.

Russia, which currently holds the G8 rotating leadership, had been scheduled to chair a summit in Sochi in June but this is unlikely to take place.

Japanese Foreign Ministry spokesman Masaru Sato disclosed that Tokyo had imposed its own sanctions, suspending talks on investment, space exploration, military co-operation and on relaxing visa requirements.