(AP) -- Russia's Foreign Ministry on Monday denounced what it called lawlessness by far-right activists in eastern Ukraine, a statement likely to trigger alarms in Ukraine about possible Russian intervention there.

Ukraine's foreign minister said Monday his country already feels like it's almost in a state of war after Russian forces took effective control of Ukraine's Crimean Peninsula. A referendum has been called there for Sunday on whether the region should split off and seek to become part of Russia.

Pro-Russia sentiment is also high in Ukraine's east and there are fears Russia could seek to incorporate that area as well.

In its Monday statement, the Russian Foreign Ministry said lawlessness "now rules in eastern regions of Ukraine as a result of the actions of fighters of the so-called 'Right Sector' with the full connivance" of Ukraine's new authorities.

Right Sector is a grouping of several far-right and nationalist factions. Its activists were among the most radical and confrontational of the demonstrators in the Ukrainian capital, Kiev, and organized "self-defense" brigades for the protest camp

The Kremlin statement also claimed Russian citizens trying to enter Ukraine have been turned back at the border by Ukrainian officials.

On Sunday, a pro-Russian crowd in the eastern city of Luhansk occupied the regional government headquarters, raised the Russian tricolor and demanded the right to hold a referendum on joining Russia, like in Crimea.

On Monday in Kiev, Ukrainian Foreign Minister Andriy Deshchytsya received his counterparts from Belgium, the Netherlands and Luxembourg, who had come to show support for Ukraine in what has turned into Europe's greatest geopolitical crisis since the end of the Cold War.

"We have to admit that our life now is almost like ... a war," Deshchytsya said, speaking in English. "We have to cope with an aggression that we do not understand."

He said Ukraine is counting on help from abroad to deal with its giant neighbor to the east.

On Wednesday, Prime Minister Arseniy Yatsenyuk will be received in Washington by President Obama, the White House has announced.

Obama has warned that the March 16 vote in Crimea would violate international law. But on Sunday, Russian President Vladimir Putin made it clear that he supports the referendum, in phone calls with German Chancellor Angela Merkel and British Minister David Cameron.

"The steps taken by the legitimate leadership of Crimea are based on the norms of international law and aim to ensure the legal interests of the population of the peninsula," said Putin, according to the Kremlin.

On Monday, Putin was briefed by Russia's foreign minister, Sergey Lavrov, on the contents of a document Lavrov received from Secretary of State John Kerry explaining the U.S. view of the situation in Ukraine, the official Itar-Tass news agency said.

That document contains "a concept which does not quite agree with us because everything was stated in terms of allegedly having a conflict between Russia and Ukraine and in terms of accepting the fait accompli," Lavrov reportedly said. By that, he meant the "coup" the Russians contend ousted Ukraine's legally elected and pro-Kremlin president, Viktor Yanukovych.

Lavrov said Kerry had been invited to come to Russia to discuss the situation. "We suggested that he come today, I think, and we were prepared to receive him. He gave his preliminary consent. He then called me on Saturday and said he would like to postpone it for a while," the minister said, according to Itar-Tass.

In Kiev, Mikhail Khodorkovsky, the businessman who was once Russia's most famous prisoner, said Monday his country is ruining its longstanding friendship with Ukraine by its aggressive and pro-separatist actions in Crimea.

"The question of Crimea's fate is very painful both for Ukrainians and for Russians. It's not just a simple territorial dispute for some extra square kilometers" Khodorkovsky told a packed hall at Kiev Polytechnic University.

"For Russians, it's a sacred place, an important element in our historical memory and the most painful wound since the Soviet collapse," Khodorkovsky said. Nevertheless, he said, the symbolism of Crimea for Russians cannot justify "such a blatant incursion into the affairs of a historically friendly state."

He called for Crimea to remain part of Ukraine, but with broader regional powers and the protection of the rights of Russian speakers there.

Khodorkovsky, once Russia's wealthiest man, was pardoned last December by Putin. Many believe he was convicted of tax violations and other crimes and sent to prison on trumped-up charges.

On Sunday, Khodorkovsky almost wept as he urged a large crowd in Kiev's center not to believe that all Russians support their government's actions in Crimea.

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Heintz reported from Moscow.