The State Department said Thursday that the extradition by Thailand of an alleged international arms trafficker is a matter of the highest priority to the United States. A Thai appeals court is expected to issue a final decision in the case of Russian businessman Viktor Bout on Friday.

Officials at the department decline to say what the impact will be on U.S.-Thai relations if the court ruling goes against the United States.

But they say the matter is of the highest priority to the Obama administration and the subject of senior diplomatic contacts in both Bangkok and Washington.

The United States is seeking the extradition from Thailand of Russian businessman Viktor Bout, an alleged international arms trafficker who has been jailed in Thailand for more than two years.

Bout was arrested in Thailand in 2008 by Thai police and U.S. drug agents for alleged involvement in a scheme to provide Russian shoulder-fired anti-aircraft missiles to Colombia's leftwing FARC guerrillas, listed by the United States as a terrorist group.

A Thai court a year ago rejected a U.S. extradition request on grounds that FARC, the Revolutionary Armed Forces of Colombia, is not a terrorist group. An appeals court has been reviewing the case and is expected to give a verdict Friday.

At a news briefing, State Department Spokesman P.J. Crowley said the United States is following the case closely and told the Thai government again this week of the importance it attaches to the case. "The extradition process is reaching a potential  conclusion, and we await the judgment of the Thai government and the Thai courts We believe that we presented significant evidence to justify his extradition to the United States. We think we've made our case, and well see what the judgment of the court is," he said.

Moscow has pushed for Bout's release, with the Russian parliament in 2008 condemning his arrest and calling for his release and return home.

The Bout case has also been an issue in the U.S. Congress.

Key legislators, including House Foreign Affairs Committee Chairman Howard Berman, warned Thailand in a letter released Thursday that its handling of the case has not been fair or transparent, and that Bout's release could harm bilateral relations.

A senior Republican member of the Foreign Affairs Committee, Ed Royce, said in a Washington Post commentary Thursday that Bout has sold arms to various factions in the conflicts in the Democratic Republic of the Congo and Afghanistan, and to former Liberian President Charles Taylor, now facing war crimes charges.

Congressman Royce said that if freed, there is little doubt that Bout will return to the illicit arms trade as a vengeful enemy of the United States. He said his release would be a miscarriage of justice that would severely harm relations with Thailand, a key U.S. Asian ally.