In an interview with Sveriges Radio's Ekot news programme Bildt dismissed parallels drawn with Kosovo, which broke away from Serbia and has been recognized by Sweden and about half of the UN member states.
"There are crucial differences. After a major war in 1999 Kosovo was placed under UN administration, a long political process underlying its independence, with the EU, Kosovo and Serbia all having participated. Crimea in contrast, has been subject to Russian military intervention, TV stations have been closed among other things," Bildt said.
"This could have been an internal political process in Ukraine but that is not the case. There is a risk of an invasion by forced annexation".
Bildt forecast that the prospect of sanctions against key Russian leaders now appeared likely when the EU foreign ministers meet as scheduled in Brussels on Monday.
"We pinned some hope to the meeting held in London yesterday between the Russian and US Secretary of States. It lasted six hours, so it was a very detailed discussion. But towards the end of this it was evident that the Russian position has not changed in any part," he told Ekot.
Bildt furthermore declined to "play with Russian propaganda" by commenting on Svoboda, the party known to have anti-Semitic and fascist elements and which has formed part of the acting Kiev government.
"We accept the government that has been formed, with many different people. It's a fair and democratic government," he said.