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.
Later, EU foreign policy chief Catherine Ashton announced she would meet with Kerry, Russian Foreign Minister Sergei Lavrov, and Ukraine's Acting Foreign Minister Andriy Deshchytsya next week, although the date and location remain unclear.
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.
Ukraine's State Security Service (SBU) said "separatists" in Luhansk had placed explosives in the state security building they seized and were holding 60 people hostage. The activists denied the claim.
Pro-Russian protesters remained in control of the regional government buidling in Donetsk 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.
In a related development, U.S. Vice President Joe Biden has discussed the crisis in Ukraine with the leaders of Montenegro and Slovakia.
Biden met at the White House on April 8 with Montenegro's prime minister, Milo Djukanovic. The White House says Biden and Djukanovic discussed the situation in the Balkans as well as Russia's pressure on Ukraine.
Biden praised Montenegro for making progress toward joining NATO and said the United States supports Montenegro's aspirations to form closer ties with Europe.
The White House says Biden also spoke by phone on April 8 with Prime Minister Robert Fico of Slovakia. The leaders discussed concerns about Russia violating Ukraine's sovereignty and upcoming elections in Ukraine.
Biden thanked Slovakia for helping Ukraine diversify its energy supply. Western nations see that as key to reducing Europe's dependence on Russian gas.
Based on reporting by Reuters, AFP, and AP