EXCLUSIVE: Sen. Joni Ernst, of Iowa, is leading the charge of five Republicans calling on the Biden administration to enact sanctions against and investigate the Wagner Group amid reports that hundreds of Russian mercenaries are in Kyiv on a mission to assassinate Ukrainian President Volodymyr Zelenskyy.

In a letter to Treasury Secretary Janet Yellen, Secretary of State Antony Blinken, and Secretary of Defense Lloyd Austin, Ernst and her colleagues asked for "immediate action" be taken to investigate reports suggesting that "Russian President Vladimir Putin has hired hundreds of mercenaries from Russia’s Wagner Group to assassinate Ukrainian President Volodymyr Zelenskyy." 

They also asked for economic sanctions imposed through the Department of Treasury’s Office for Foreign Assets Control (OFAC) "pursuant to Executive Order 14024 on the Wagner Group, including Yevgeny Prigozhin, Dmitry Utkin, and Konstantin Pikalov, as well as any other known affiliates."

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"According to the reports, the Wagner Group entered Ukraine at the direction of Russian President Vladimir Putin with the goal to assassinate Ukrainian President Volodymyr Zelenskyy to undermine the peace, security, political stability, and territorial integrity of Ukraine as part of the Russian invasion," the letter asserted. "We owe it to our allies to punish private actors to the greatest extent possible that directly participate in Russia’s warmongering in Ukraine and we urge you to take all necessary measures to hold them accountable."

WASHINGTON, DC - JANUARY 19: Sen. Joni Ernst (R-IA) speaks during a press conference following the weekly Senate Republican policy luncheon in the Russell Senate Office Building on Capitol Hill on January 19, 2022 in Washington, DC. During the news conference, McConnell, along with Senate Republican Leadership, spoke on their views of the attempts of Senate Democrats to pass further voting rights legislation and rules to amend the filibuster. (Photo by Anna Moneymaker/Getty Images)

Sen. Joni Ernst (R-IA) speaks during a press conference following the weekly Senate Republican policy luncheon in the Russell Senate Office Building on Capitol Hill on January 19, 2022, in Washington, DC.  (Anna Moneymaker/Getty Images)

The Wagner Group, technically a private Russian military company, has been considered a proxy group of the Russian state abroad since its inception in 2014 and participated in the annexation of Crimea.

In December, the European Union, with support from the State Department, enacted sanctions against the Wagner Group and its associates for serious human rights abuses, including torture and extrajudicial killings in countries including Libya, Syria, the Central African Republic and Ukraine’s Donbas region.

The paramilitary organization was founded by Utkiny, a veteran of both Chechen wars and a former member of the Main Intelligence Directorate, or the Russian intelligence agency abbreviated as GRU. 

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An alleged close ally to Putin, Russian oligarch Prigozhin has been accused of financially backing the Wagner Group. He is wanted by the FBI for allegedly attempting to interfere with U.S. elections from early 2014 to 2018 through St. Petersburg-based Internet Research Agency, dubbed the "troll factory."

Pikalov is a colonel who’s been accused of heading the Wagner Group’s operations in Africa. 

Ukrainian President Volodymyr Zelenskyy

Ukrainian President Volodymyr Zelenskyy stands alongside other government officials in a video posted to social media Friday vowing to defend the country from a Russian invasion.  (Armed Forces of Ukraine)

Ernst, a combat veteran and member of the Senate Armed Services Committee, signed the letter dated March 2 and exclusively obtained by Fox News Digital on Thursday. It was signed by Sens. Charles Grassley, R-Iowa, Mike Braun, R-Indiana, Lindsey Graham, R-S.C., and Bill Hagerty, R-Tenn.

Last April, Biden issued an executive order targeting the Russian Federation for broad sweeping offenses, which included undermining the conduct of free and fair democratic elections and democratic institutions in the U.S., issuing malicious cyber activity against the U.S. and its allies, and threatening "well-established principles of international law, including respect for the territorial integrity of states."

The letter notes that Biden said in his order that "persons responsible for or complicit in, or have directly or indirectly engaged or attempted to engage" in the "assassination, murder or other unlawful killing of, or infliction of other bodily harm against, a United States person or a citizen or national of a United States ally or partner; activities that undermine the peace, security, political stability, or territorial integrity of the United States, its allies, or its partners" are eligible for expansive economic sanctions. 

This comes as a convoy of tens of thousands of Russian troops has been stalled outside Kyiv. Zelenskyy reportedly turned down an evacuation offer from the U.S. and vowed to fight beside his countrymen. Russian forces have gained control of the Black Sea port of Kherson, making it the first city to fall since the invasion of Ukraine began over a week ago. There’s been a mass evacuation of Ukraine’s second-largest city of Kharkiv, which has been battered heavy shelling over the past several days. 

The Wagner Group is not the only organization allegedly plotting Zelenskyy’s assassination. 

Fox News Digital previously reported that Putin sent the battle-hardened and much-feared Chechen fighters into Ukraine to support the Russian invasion, possibly on a mission to kill or capture senior Ukrainian politicians. Ukrainian security forces "eliminated" a hit squad of elite Chechen commandos sent to murder Zelenskyy, a top Kyiv defense officials told national media earlier this week. 

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The Times of London first reported on the alleged mission of the Wagner Group, saying more than 400 mercenaries were flown in from Africa over the past five weeks with orders to decapitate Zelenskyy’s government and make way for Russian forces to establish control of the Ukrainian capital. 

A senior U.S. defense official told reporters Monday that there are reconnaissance forces in Kyiv but could not confirm those forces were ordered to assassinate Zelenskyy. 

Kyiv Mayor Vitali Klitschko had issued a nightly curfew, warning that any residents who go outside could be mistaken for enemy sabotage and reconnaissance groups.