That claim is limited to the interim government in Kiev. They WISH it were the case.
Sir Tony Brenton, who served as British Ambassador from 2004 to 2008, said that war could be an option 'if we do conclude the [Budapest] Memorandum is legally binding.' It promises to protect Ukraine's borders, in return for Ukraine giving up its nuclear weapons. Kiev has demanded the agreement is activated after insisting their borders had been violated. In response Mr Brenton said in a BBC radio interview: 'If indeed this is a Russian invasion of Crimea and if we do conclude the [Budapest] Memorandum is legally binding then it's very difficult to avoid the conclusion that we're going to go to war with Russia'.
From link: Budapest Memorandum” Treaty Signed In 1994 May Commit U.S., Britain To Defend Ukraine