"It's difficult to rank these things but it's certainly the biggest crisis in Europe in the 21st century and it will require all our diplomatic efforts," Hague said from Kiev, where he is meeting the interim Ukrainian government. (Agencies)
The British Foreign Minister urged Russia to pull back its forces in Crimea or face "significant costs".
Referring to the decision by Britain and other allies to pull out of preparatory talks on the G8 summit in Sochi this week, Hague said, "There are diplomatic measures which we have started on already. There are a range of other significant costs. I don't want to anticipate at the moment what those will be; those will be discussed among my fellow EU Foreign Ministers on Monday. They are also for discussion with the United States, Japan, Canada, other nations.”
"But be in no doubt that there would be such costs. The world cannot just allow this to happen. The world cannot say it's OK in effect to violate the sovereignty of another nation in this way," he added.
US Secretary of State John Kerry warned on Sunday that Russia risked losing its seat among the prestigious Group of Eight nations, although Hague did not go that far.
"What we will do in subsequent weeks depends on Russia's behaviour. Clearly that means there is a serious threat to G8 cooperation over the coming weeks and months," Hague said.
"The G7 countries are entirely capable of cooperating well among themselves without Russia and they will increasingly move in that direction if these matters cannot be resolved."
Explaining what Western powers wanted Moscow to do, Hague said Russia was entitled to have forces in Crimea.
"But when they are outside their bases they are meant to operate with the agreement of the Ukrainian authorities. Russia needs to return to that situation," he said.
"It's difficult to rank these things but it's certainly the biggest crisis in Europe in the 21st century and it will require all our diplomatic efforts," Hague said from Kiev, where he is meeting the interim Ukrainian government.