- U.S. and NATO say Russia's SSC-8 missile violates terms of the 1987 missile treaty.
- The U.S. has now ended its participation in the Reagan-era treaty.
- NATO has promised measures to protect countries that border Russia from the missile threat.
The U.S. has formally withdrawn from a decades-old Intermediate-Range Nuclear Forces treaty, which banned ground-launched medium-range missiles with a range of 310 to 3,400 miles.
The 1987 Reagan-era treaty ended after Moscow refused to destroy its new intermediate-range SSC-8 cruise missile that Washington said violated the INF.
The SSC-8 missile can be tipped with conventional or nuclear explosives.
President Donald Trump announced in February that he would end the agreement unless Russia mothballed the missile system. At the time, Trump added that if Russia didn't comply, the U.S. would "move forward with developing our own military response options" to Russia's SSC-8.
In a statement Friday, which confirmed the U.S. withdrawal, Secretary of State Mike Pompeo said Russia was "solely responsible for the treaty's demise" and that U.S. efforts to seek compromise had been continuously rejected by the Kremlin.
"Dating back to at least the mid-2000s, Russia developed, produced, flight tested, and has now fielded multiple battalions of its noncompliant missile. The United States first raised its concerns with Russia in 2013. Russia subsequently and systematically rebuffed six years of U.S. efforts seeking Russia's return to compliance."
The statement was released at the same time as a tweet from Pompeo's Twitter account.
Russia's short- and medium-range missiles are viewed as a particular threat to its neighboring countries as the weapons can be quickly launched, leaving the target country or region with almost no response time.
Pompeo's statement said fielding the SSC-8 missile system in sites around Russia represented "a direct threat to the United States and our allies and partners."
NATO added its weight to the U.S. position Friday, stating that Russia "bears sole responsibility" for the treaty collapse and that the military alliance would now respond in a "measured and responsible way" to "risks posed by the Russian 9M729," an alternative name for the SSC-8.
NATO said in June that Russia must dismantle the short-range system, or the Western alliance would be forced to respond, adding that NATO defense ministers would now look at next steps "in the event that Russia does not comply."
Responding to that June threat, Russia promised to take "countervailing military measures."
CNBC reached out to Russian President Vladimir Putin's press office but a comment was not immediately available.