The Obama administration and a half-dozen members of Congress are expressing concern over the potential release of reputed international arms merchant Viktor Bout by the government of Thailand.

On Thursday, the State Department said that sending the Russian businessman to the United States to face criminal charges is a high priority for the U.S.

State Department spokesman P.J. Crowley said Thursday that the Thai ambassador in Washington was summoned to the State Department this week so that U.S. officials could "emphasize that this is of the highest priority to the United States."

Crowley said the U.S. ambassador in Bangkok has also met with officials with the Thai foreign ministry to deliver the same message.

The head of a lucrative air transport empire, Bout has long evaded accusations by the U.S. and the U.N. of violating arms embargos despite being targeted with harsh financial sanctions aimed at scuttling his network. His 2008 arrest by Thai authorities landed him in prison for the first time and set off a legal tug-of-war between the U.S. and Russia.

The United States has "presented significant evidence to justify" Bout's extradition to the United States, Crowley said. "We have made our case, and we will see what the judgment of the court is," he added.

In a separate letter Wednesday to Thailand's ambassador in Washington, three Democrats and three Republicans in Congress said that if Bout is freed, he would provide arms to groups that target U.S. and Thai interests around the globe.

After Bout was arrested in March 2008 during an arms sting by U.S. narcotics agents and Thai police, federal prosecutors indicted him for aiding a terror group and pressed for his extradition to New York to stand trial. Russia quickly objected and demanded Bout's release.

The U.S. and Russia are awaiting a court ruling Friday in Thailand on the matter.

"We find the potential release of a man responsible for countless deaths of innocents in Africa and elsewhere simply astounding," the lawmakers wrote.

The six lawmakers are Reps. Howard Berman, D-Calif., and Ileana Ros-Lehtinen, R-Fla., the chairman and ranking Republican, respectively, on the House Foreign Affairs Committee; Reps. Brad Sherman, D-Calif., and Ed Royce, R-Calif., the chairman and ranking Republican, respectively, on the House Subcommittee on Terrorism, Nonproliferation and Trade; Sen. Russ Feingold, D-Wis., chairman of the Senate Subcommittee on African Affairs; and Rep. Frank Wolf, R-Va., the ranking Republican on the House Appropriations Subcommittee on Commerce, Justice and Science.