Pro-Russian forces have taken control of a Ukraine naval base in Crimea.
Moscow is racing to incorporate the region into Russia, ignoring the threat of sanctions from Washington.
The U.S. and its European allies have already frozen assets and slapped visa bans on some Russian and Crimean officials.
Russian President Vladimir Putin signed a treaty with Crimean leaders following this weekend's vote for Crimea to join the country. Russian lawmakers plan to make the treaty official by the end of the week.
Vice President Joe Biden is wrapping up a trip to the region.
"I want to make it clear: we stand resolutely with our Baltic allies in support of the Ukrainian people and against Russian aggression," Biden said.
Biden says Putin is on a "dark path toward isolation".
Ukrainian's prime minister called together his cabinet while Ukrainian troops on high alert conducted drills near Russia's border.
Britain's Prime Minister warns if more steps are taken Russia could be expelled from the G8.