Russia's takeover of Crimea could prompt a review the US military presence in Europe, which has declined steadily since the end of the Cold War.
"While we do not seek confrontation with Russia, its actions in Europe and Eurasia may require the United States to re-examine our force posture in Europe and our requirement for future deployments, exercises, and training in the region," Assistant Secretary of Defense Derek Chollet said on Tuesday.
Some 67,000 US military members are currently stationed on the European continent, mainly in Germany (40,000), Italy (11,000) and Britain (9,500).
When the Soviet Union fell in late 1991, the total presence stood at 285,000.
Chollet, testifying before the House Armed Services Committee, did not specify what such a re-examination could entail at a time when the Pentagon faces budget cuts and is seeking to redeploy part of its resources to the Asia Pacific region as part of a so-called pivot strategy.
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"Russia's unlawful military intervention in Ukraine challenges our vision of a Europe whole, free and at peace," he said.
"It changes Europe's security landscape. It causes instability on NATO's borders. And it is a challenge to the international order."
To reassure Eastern European NATO members, Washington has already deployed six F-15s as reinforcement to the Baltics, as well as 12 F-16s and three transport planes to Poland.
A guided-missile destroyer, the USS Donald-Cook, is due to arrive in the Black Sea in the coming days.
The seizure of local administration buildings in Donetsk and Kharkiv in eastern Ukraine was "very concerning," Chollet said, adding that Washington did not believe them to be "spontaneous demonstrations."
"Moving into eastern Ukraine would clearly be a very serious escalation of this crisis," he said.
