Riot police and other Ukrainian forces are cracking down on pro-Russian demonstrators in eastern Ukraine, drawing a warning from neighboring Russia on Tuesday that also alleged an American military contractor is helping Ukraine.

Interior Minister Arsen Avakov says Ukraine has arrested around 70 demonstrators who had seized a regional administration building in Kharkiv, the country's second-largest city. Avakov described it as an "anti-terrorist" operation.

The crackdown drew a response from Moscow, as the Los Angeles Times reports. Here's the newspaper's account of the warning issued by Russia's Foreign Ministry:

"According to our information, Ukraine Interior Ministry and National Guard troops including militants of the illegal armed group the Right Sector are being brought to the southeast regions of Ukraine," read a statement posted on the Russian Foreign Ministry's official website Tuesday. "A special concern is connected with the fact that about 150 U.S. experts from the private military organization Greystone dressed in the uniforms of [Ukraine] special unit Sokol are involved in the operation."

A representative from Greystone, based in Chesapeake, Va., told The Wall Street Journal that the company was not involved. "We do not have anyone working in Ukraine nor do we have any plans to deploy anyone to the region," Coreena Taylor said.

Ukraine's interim presidential chief of staff, Serhiy Pashynsky, claimed the demonstrators were being financed by ousted President Viktor Yanukovych, according to the Kyiv Post. Yanukovych is now living in Russia.

As we reported Monday, separatists also have seized a government building in the city of Donetsk, where they declared an independent "people's republic."

"Police say that in a third protest in the city of Luhansk pro-Russia activists inside the main state security building have seized weapons," Reuters says.

As tensions have risen in recent days, Ukraine's richest man has been urging the separatists to stand down. NPR's Ari Shapiro reports on that angle of the story for our Newscast unit:

"Rinat Akhmetov is a multibillionaire who owns many of Ukraine's steel mills and coal mines. Now he seems to be acting as a mediator with pro-Russian activists.

"A YouTube video shows him at the scene of protests in the Eastern Ukrainian city of Donetsk, where he appears to express sympathy with the demonstrators.

"In the video, the oligarch seems to encourage the separatists to discuss their concerns — adding that he shares the worries of Russian speakers in the region, but breaking off from Ukraine is not an option.

"This follows days of protests in Eastern Ukrainian cities. Leaders of Ukraine's interim government accuse Russia of provoking the demonstrations in order to destabilize the country. Russia dismisses those claims."

On Tuesday, NATO Secretary-General Anders Fogh Rasmussen called on Russia to "step back" to ease tensions in the region. That line was part of a speech delivered in Paris; here's the relevant passage:

"Events in Eastern Ukraine are of great concern. I urge Russia to step back. Any further move into Eastern Ukraine would represent a serious escalation, rather than the de-escalation that we all seek. We call on Russia to pull back the tens of thousands of troops it has massed on Ukraine's borders, engage in a genuine dialogue with the Ukrainian authorities, and respect its international commitments.

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