U.S. Secretary of State John Kerry says Russian agents have been sent to eastern Ukraine to create "chaos."
Kerry also said that Russia's actions in eastern Ukraine could be a contrived pretext for military intervention just like in Crimea.
Kerry was testifying before a U.S. Senate panel in Washington on April 8.
He said, "Everything that we've seen in the last 48 hours, from the Russian provocateurs and agents operating in eastern Ukraine, tells us that they've been sent there determined to create chaos."
Kerry called these efforts "illegal and illegitimate."
Kerry said the United States and its European allies were willing to impose tough new sanctions on Russian energy, banking, and mining entities if Russia continues to infringe on Ukraine's sovereignty.
Kerry also said he would meet with Russian Foreign Minister Sergei Lavrov and with Ukrainian officials next week in Europe.
NATO Secretary-General Anders Fogh Rasmussen earlier on April 8 warned Russia that any incursion into eastern Ukraine would be a "historic mistake."
The comments come as Ukrainian authorities were moving to counter pro-Russian protesters who seized government buildings in three cities in eastern Ukraine.
Authorities in Kyiv said they seized back control of the regional administration building in Kharkiv from pro-Moscow activists, and arrested about 70 "separatists."
In two other eastern cities, Donetsk and Luhansk, the standoff continues.
In Luhansk, protesters occupying the state security building have blocked neighboring streets with barbed wire and car tires.
In Donetsk, a group of pro-Moscow activists remain inside the regional government building where on April 7 they declared a "sovereign people's republic."
In Moscow, Russia's Foreign Ministry warned in a statement that any use of force against pro-Russian protesters in the east could drag Ukraine into civil war.
Russian Foreign Minister Sergei Lavrov said the situation in Ukraine will improve only if concerns of Ukraine's Russian-speakers are addressed.
Based on reporting by Reuters, AFP, and AP
We would like to hear your opinion about the new site. Tell us what you like, and what you don't like in an email and send it to: firstname.lastname@example.org