NATO members are worried that unprecedented billion-dollar arms sales to Russia by France, Germany and Italy could destabilize security, a US congressional report said Thursday 10 days before the NATO summit.

The report by the Congressional Research Service details the sales to Russia by France of four of its Mistral-class amphibious assault vessels, which CRS described as "the first ever (sales) of a significant offensive military capability by a NATO member to Russia."

The sales, beginning with a France-Russia contract signed in June 2011, have "exposed tension within the alliance over NATO's relations with Russia" and led to particular concern among Baltic and other NATO members about possible deployment of the Mistrals in the Baltic Sea.

US President Barack Obama's administration opposed the sales, the report said, because they "could send the wrong message both to Russia and to some Central and Eastern European allies."

But Washington did not speak out forcefully against the sales because of the priority it has placed on improving ties with Moscow, the report cited analysts as saying.

Obama's administration launched a "reset" in US-Russia in 2009 and has been "a proponent of extending this policy approach to the NATO-Russia relationship," the study said.

CRS produced the 31-page report on request from senior Republican Senator Richard Lugar, who had expressed concern that the weapons could be used against US allies and that sales might one day expand to China.

The Mistral, the second largest ship in the French navy, is a force projection vessel that can transport up to 16 helicopters, four landing craft, 13 battle tanks, and hundreds of combat troops, and can field a 69-bed hospital.

Russia will pay $1.47 billion for the first two vessels, and France's state-owned naval defense company DCNS said it will deliver the first ship to Russia in 2014, CRS said.

German defense giant Rheinmetall signed a deal with Russia's Defense Ministry last November to build a $131 million army training center in Russia's Volga region which Rheinmetall describes as "the most advanced system of its kind worldwide," CRS said.

Italy reached agreement with Russia's Defense Ministry for the sale of dozens of light multirole armored vehicles manufactured by a Fiat subsidiary.

Despite concerns by some North Atlantic Treaty Organization members that the overall sales could lead to regional destabilization, analysts and diplomats concur that the sales do not represent a severe military threat by Russia.

"French, German, and Italian officials stress that recent military sales to Russia should be viewed as a logical step in advancing a broader political goal of strategic partnership with Russia," the report said.

Obama hosts the NATO summit May 20-21 in Chicago.