The U.S. formally accused Russia of violating a 1987 treaty that banned ground-to-air cruise missile testing, which the U.S. says Russia has been doing since 2008.

The letter from the Obama administration to the Putin administration comes at a time when tensions between the two have already escalated amid the Ukraine conflict and Russia's protection of NSA whistleblower Edward Snowden, according to The New York Times.

A report about Russia's actions was supposed to be released in April but has been released now, according to the The Associated Press.

The accusation also comes at a time when the European Union and U.S. are introducing tougher sanctions against Russia, who is retaliating with plans to ban companies from "aggressor states" from doing business, according to The Moscow Times.

Earlier this year, the U.S. informed NATO that Russia had violated the Intermediate-range Nuclear Forces Treaty (INF) by testing the missiles.

"The United States has determined that the Russian Federation is in violation of its obligations under the I.N.F. treaty not to possess, produce or flight test a ground-launched cruise missile (GLCM) with a range capability of 500 kilometers to 5,500 kilometers or to possess or produce launchers of such missiles," a report by the State Department to be released soon will say.

And since reports state that Russia has been testing such missiles since 2008, as recently as 2011, NATO has replied that a response to such violations will be necessary.

"A weapon capability that violates the I.N.F., that is introduced into the greater European land mass, is absolutely a tool that will have to be dealt with," NATO's top commander, Gen. Philip M. Breedlove, told The New York Times in April. "It can't go unanswered."

Russia responded to the allegations by pointing to the U.S. use of the Aegis missile system, which will be based in Romania, and will be used to protect U.S. and NATO forces from missile attacks. Both sides are still interested in discussing the issues, however, and have reported they intend to continue talks.

Russian media have not reported on the accusation, but one reported an official's comments on relations between the two nations Tuesday, according to RT.com.

Deputy foreign minister Sergei Lavrov said, "It is unlikely, because Washington has gone too far in its anti-Russian rhetoric and the constant urge to punish us for something, now it will be difficult to 'rewind' the situation."

But the U.S. is pursuing the treaty violation.

"This is a very serious matter which we have attempted to address with Russia for some time now," an administration official said in a statement, according to the Guardian. "We encourage Russia to return to compliance with its obligations under the treaty and to eliminate any prohibited items in a verifiable manner."