It was announced UK ministers would boycott the Paralympic Games in Russia. Britain is also boycotting G8 preparatory meetings due to take place in Russia. It came as Nato's head warned European peace was at risk.
Ukraine's acting president Arseniy Yatsenyuk said his country "stood on the brink of disaster" as its neighbour continued to build up forces in the Crimea region.
Russia's premier Vladimir Putin has ignored calls from the US to withdraw troops. Meanwhile, a huge crowd gathered in Kiev in protest against the build-up, bearing "Putin Stop" placards.
Nato chief Anders Fogh Rasmussen opened crisis talks to forge a joint response to Moscow's threat of military intervention. "Russia must stop its military activities," said Mr Rasmussen. US Secretary of State John Kerry branded the Russian move as "an incredible act of aggression".