WASHINGTON ― A bipartisan group of U.S. lawmakers is pressuring the Trump administration to release $250 million in military aid to Ukraine.

In a letter sent Tuesday to acting White House Chief of Staff Mick Mulvaney, the leaders of the Senate’s bipartisan Ukraine Caucus expressed “deep concerns” as the Trump administration evaluates the aid package.

Vice President Mike Pence, a day earlier in Warsaw, Poland, voiced U.S. support for Ukraine and condemned Russia’s “illegal occupation of Crimea and Russian aggression in eastern Ukraine.” But Pence also expressed concerns about Ukraine’s corruption problems and seemed to confirm that President Donald Trump asked that the aid package be reviewed.

“I mean, to invest additional taxpayer in Ukraine, the president wants to be assured that those resources are truly making their way to the kind of investments that will contribute to security and stability in Ukraine,” Pence told reporters. “And that’s an expectation the American people have and the president has expressed very clearly.”

Pence said that when he and Ukrainian President Volodymyr Zelensky met in Warsaw, they discussed America’s support for Ukraine and “the upcoming decision the president will make on the latest tranche of financial support, in great detail.”

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The move to hold up the aid, which helps Ukraine buy lethal weapons and was first reported by Politico last week, sparked bipartisan ire. Lawmakers argue the U.S. must maintain its commitment to help Ukraine defend against Russia-backed separatists, since Russia annexed Crimea in 2014.

The aid “has helped Ukraine develop the independent military capabilities and skills necessary to fend off the Kremlin’s continued onslaughts within its territory,” the lawmakers wrote. The funding falls under the Ukraine Security Assistance Initiative.

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Tuesday’s letter to Mulvaney comes from Ukraine Caucus co-chairs Rob Portman, R-Ohio, and Dick Durbin, D-Ill., as well as Sens. Ron Johnson, R-Wis.; Dick Blumenthal, D-Conn.; and Jeanne Shaheen, D-N.H. Johnson chairs the Senate Foreign Relations Subcommittee on Europe and Regional Security Cooperation, and Shaheen is its top Democrat.

“U.S.-funded security assistance has already helped turn the tide in this conflict, and it is necessary to ensure the protection of the sovereign territory of this young country, going forward,” they wrote.

The letter follows separate pushback on social media from Senate Minority Leader Chuck Schumer, D-N.Y., and the top Democrat on the Senate Foreign Relations Committee, Sen. Bob Menendez of New Jersey.

"In willfully delaying these funds, the Trump Administration is once again trying to circumvent Congress' Constitutional prerogative of appropriating funds for U.S. government agencies,” Menendez said in a statement. “It is also undermining a key policy priority that has broad and deep bipartisan support.”

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Rep. Adam Kinzinger, the top Republican on the House Foreign Affairs subcommittee on Europe, wrtoe in a tweet Thursday: "This is unacceptable. It was wrong when [President Barack] Obama failed to stand up to [Russian President Vladimir[ Putin in Ukraine, and it’s wrong now.”

The Trump administration’s reported request for senior officials ― Defense Secretary Mark Esper and national security adviser John Bolton ― to review the aid package became public after the White House lost an internal battle to cut $4 billion foreign aid.

U.S. lawmakers have urged Trump to take a harder line on Russia on a number of fronts since he took office. The latest controversy comes days after Trump campaigned to reinstate Russia into the Group of Seven, or G-7, during its annual gathering in Biarritz, France. Russia was excluded in 2014 after its annexation of Crimea.

In response to questions Tuesday, a State Department official emphasized that U.S. policy supporting Ukraine in the face of Russian aggression remains unchanged and that the U.S. has committed more than $1.5 billion in aid over time. Bolton visited Kyiv just days ago, while Secretary of State Pompeo issued a recent statement of support.