Russian military
An armed Russian soldier provides security as Russian tanks arrive at a train station in the Crimean settlement of Gvardeiskoye near the city of Simferopol on March 31, 2014. Reuters/Yannis Behrakis

Russia’s Federal Security Service, or FSB, on Thursday arrested 25 Ukrainian nationals on suspicion of plotting acts of terror and sabotage that the agency claimed could have led to hundreds of deaths in the country, news reports said.

The FSB reportedly said in a statement that these individuals were plotting terror attacks across seven different regions in Russia, Associated Press, or AP, reported citing Ria Novosti, a Russian news agency. Three activists from Right Sector, a Ukrainian nationalist group, were also among those arrested by the agency, who, according to the FSB's claims, were instructed by Ukraine’s Security Service to obtain pictures of Russia’s military movements on the border, AP reported, citing Russian news channel NTV. The arrested people were also instructed to establish contact with Russian radical groups, the FSB claimed.

Russia, which is said to have deployed 40,000 troops near its border with Ukraine, was accused Thursday by Ukrainian officials of sending FSB operatives armed with explosives into Kiev to deter anti-government protests.

While the presence of troops near the border has led NATO to fear a Russian attack on Ukrainian soil, the U.S. Department of Energy, on Thursday, reportedly suspended operations with its Russian counterpart on experimental projects on nuclear energy, according to Ria Novosti.

"The US side has informed us about a freeze on some joint projects in the sphere of peaceful nuclear energy. We are talking specifically about a series of technical meetings, including scientific ones," Ria Novosti reported, citing a source from the state-run nuclear agency, Rosatom.

President Vladimir Putin's move to annex Crimea has upset western powers including the U.S. and European Union, or EU, who believe Ukraine’s sovereignty has been violated by Russia, and have responded with economic and political sanctions against Russian officials in the form of asset freezes and travel bans.

Meanwhile, agencies and western nations are also working to channel funds to Ukraine's interim government to keep the nation's economy afloat. The International Monetary Fund last week announced that it will unlock $14 billion to $18 billion in aid for Ukraine, while the U.S. approved a $1 billion loan guarantee for Ukraine.

The U.S. and the EU have also been discussing methods to diversify sourcing the latter's gas needs, but a report on Wednesday estimated that Europe would have to spend nearly $215 billion in additional investments if the region were to reduce its reliance on Russian gas, Ria Novosti reported.