The United States said Tuesday it had only low expectations for an upcoming meeting with Russia, Europe and Ukraine on the crisis in the former Soviet republic.
"I have to say that we don't have high expectations for these talks, but we do believe it is very important to keep that diplomatic door open and will see what they bring," said Victoria Nuland, assistant secretary of state for European affairs.
US and EU diplomats have agreed with Russia to hold four-way negotiations involving Ukraine next week to de-escalate the worst European security crisis in decades.
An EU diplomat said the talks would likely be held on April 17 in Vienna, but a US official, asking to remain anonymous, said they could be on April 16 in Geneva.
US Secretary of State John Kerry on Tuesday accused Moscow of sending agents into eastern Ukraine to stoke a secession crisis in ethnic Russian areas of the country.
The latest turmoil follows Russia's annexation of Crimea last month after the pro-Moscow government in Kiev fell following weeks of protests, triggering a snap referendum in Crimea to break away from Ukraine.
Kerry on Wednesday spoke by phone with Russian Foreign Minister Sergei Lavrov, and also placed a separate call to Ukrainian Prime Minister Arseniy Yatsenyuk ahead of next week's talks.
Russia was seeking to intimidate Ukrainians by massing more than 40,000 troops with quick strike aircraft along its borders and with trade blockades and gas price hikes, Nuland told US lawmakers.
"This week's violent occupations of government buildings in Karkhiv and Donetsk and in Lugansk deepen our concerns," she said.
"Far from a spontaneous set of events, as Secretary Kerry said yesterday... these incidents bear all the hallmarks of an orchestrated campaign of incitement, separatism and sabotage of the Ukrainian state, aided and abetted by the Russian security services," she said.
State Department spokeswoman Jen Psaki said the US was watching Moscow's moves as it weighs whether to tighten its sanctions on Russia.
"We look at a range of steps, including escalatory steps, and obviously their involvement through financial means and through other means, and the actions over the last couple of days have raised concerns, and they're all factors that we look at," Psaki told reporters.
But she insisted that despite the low expectations for next week's talks, other things were going on behind the scenes.
"It is not as if we are all holding our breath waiting for this meeting. This meeting is a part of many steps that we are taking in working closely with the government of Ukraine in their efforts to go through this transition period."
Nuland meanwhile insisted: "We cannot be complacent about the security of our NATO allies who live close to Russia. Our message to (President Vladimir) Putin and to Russia is clear: NATO territory is inviolable."