US Secretary of State John Kerry has flown to Kiev to support to its leaders, with Washington suspending military co-operation with Russia amid a Cold War-style struggle between Moscow and the West.
Russia has meanwhile ordered 150,000 troops who were holding military drills near the Ukrainian border back to their barracks.
Its forces remain in de facto control of Crimea - a strategic Black Sea peninsula that has housed the Russian Black Sea Fleet since the 18th century - but there were no signs on Tuesday of them conducting a military offensive overnight.
Ukrainian defence said on Monday Russia had issued its soldiers in Crimea a dawn ultimatum to surrender or face an all-out assault. Russia had denounced the claim as "complete nonsense".
Kerry, the highest profile foreign visitor to Kiev since the February 22 downfall of Kremlin-backed president Viktor Yanukovych, was to meet members of the new government as the West grapples with the biggest test for global diplomacy since the 1989 fall of the Berlin Wall.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
He will "reaffirm the United States' strong support for Ukrainian sovereignty, independence, territorial integrity, and the right of the Ukrainian people to determine their own future, without outside interference or provocation," the State Department said.
Kiev's new leaders were installed after three months of protests culminated in days of carnage that claimed nearly 100 lives and led to Yanukovych's replacement by a team seeking help from the West.
It escalated further on Monday when Washington announced a raft of tough sanctions against Russia.
