Russia has no military secrets to hide in the case of suspected arms dealer Viktor Bout and wants to ensure he has a fair trial in a US court, senior officials said on Thursday.

In a softening of Moscow's tone in a case that risks harming improved but still fragile relations with Washington, Foreign Minister Sergei Lavrov backtracked from previous heavy criticism of Bout's extradition from Thailand to the US.

"We will not act as Bout's advocates and do not claim that he did not commit any illegal offences. That we do not know, and no one will know, until justice is done," Lavrov was quoted as saying by official RIA Novosti news agency.

"We want to see justice prevail, nothing more," he said.

The remarks contrasted strongly with Lavrov's comments earlier in the week, when he said Russia would support Bout "by all means" and termed his extradition to the US as "extreme injustice."

Earlier Thursday, President Dmitry Medvedev's top foreign policy aide said Moscow wanted to see a thorough investigation into the alleged arms dealer.

"We have nothing to hide, no-one sees any military secrets or secrets of some other nature here," Sergei Prikhodko said on a visit by Medvedev to Azerbaijan.

"We are interested that the investigation into this comrade is completed and he should answer the questions that US justice has for him," he said in televised remarks. Thai authorities this week extradited Bout to the United States, after he had been held since March 2008 following a sting operation in Bangkok involving undercover US agents posing as Colombian FARC rebels
On Wednesday, the former Soviet air force pilot pleaded not guilty to terrorism charges in a New York federal court.