A U.S. State Department spokeswoman says the "evidence is overwhelming" that recent pro-Russia uprisings in eastern Ukrainian cities were a "very carefully orchestrated, well-planned, well-targeted" effort to take over buildings in several different cities in the same 24-hour period.
Victoria Nuland, the top U.S. diplomat for Europe, made the remarks when asked by a U.S. lawmaker if the U.S. has any proof that Russia was involved in the unrest.
Nuland testified Wednesday that events in Ukraine are "a wake-up call" and said "everything we have stood for over 40 years as a community of free nations" is at risk if "aggressive actions" are allowed to go unchecked and unpunished.
She said the United States does not have high expectations for upcoming four-way talks on Ukraine, but it is important to keep channels of communication open.
Earlier Wednesday, Ukraine's interior minister warned that standoffs with pro-Russian demonstrators in three eastern regions must be resolved in the next 48 hours, either through negotiations or by force.
Arsen Avakov told reporters Wednesday that a "political solution" is still possible. But he vowed protesters who want conflict "will get a forceful answer from Ukrainian authorities."
Pro-Moscow groups seized several government buildings in eastern Ukraine and demanded a vote on joining Russia, in a dramatic escalation of the protests against the country's Western-friendly interim government.
Authorities said shots were fired and at least 60 protesters arrested in Kharkiv Tuesday, while demonstrators held on to government offices captured Sunday in Donetsk.
In Luhansk, Ukrainian authorities said more than 50 people were allowed to leave state security headquarters early Wednesday where the pro-Russians were allegedly holding 60 hostages. The officials said the protesters wired the building with explosives.
On Tuesday U.S. Secretary of State John Kerry accused Russian agents and special forces of stoking separatist unrest in eastern Ukraine. He called the Russian actions "a contrived pretext for military intervention just as we saw in Crimea."
A buildup of troops along Russia's border with Ukraine has raised concerns that Moscow could move to take parts of eastern Ukraine, following its annexation of the largely pro-Russian Crimean peninsula last month.
Russia's foreign ministry on Wednesday said Moscow is not carrying out any "unusual or unplanned activity on its territory near the border with Ukraine that would be of military significance."
But many are not convinced. German Chancellor Angela Merkel told lawmakers Russia has not taken sufficient steps to ease tensions. She urged an international observer mission in Ukraine be bolstered to include 500 people.
Ukraine's pro-Russian government was ousted earlier this year following weeks of anti-government protests. It was replaced by a Western-friendly government.