U.S. Defense Secretary Jim Mattis said on Thursday that Russia's violation of an arms control treaty was "untenable" and unless it changed course the United States would need to match Moscow's capabilities.
The United States believes Russia is developing a ground-launched system in breach of a Cold War treaty, known as the Intermediate-Range Nuclear Forces Treaty (INF), that could allow Moscow to launch a nuclear strike on Europe at short notice. Russia has consistently denied any such violation.
"Russia must return to compliance with the INF treaty or the U.S. will need to match its capabilities to protect U.S. and NATO interests," Mattis said in prepared remarks to reporters during a NATO meeting of defence ministers in Brussels.
"The United States is reviewing options in our diplomacy and defense posture to do just that. Make no mistake: The current situation, with Russia in blatant violation of this treaty, is untenable," Mattis said.
The 1987 Intermediate-Range Nuclear Forces Treaty bans medium-range missiles capable of hitting Europe or Alaska and ended a Cold War-era crisis, when the Soviet Union installed nearly 400 nuclear warheads pointed at western Europe.
Mattis' comments are likely to worsen relations between Moscow and the West that are at a post-Cold War low over Russia's 2014 seizure of Crimea, its bombing campaign in Syria and accusations of meddling in Western elections.
While the United States has called for Russia to come back into compliance with the INF treaty for several years, Mattis' comments come just days after Washington's envoy to NATO said Russia must halt its covert development of a banned cruise missile system or the United States would seek to destroy it before it becomes operational.
U.S. ambassador to NATO Kay Bailey Hutchison said on Tuesday Washington remained committed to a diplomatic solution but was prepared to "take out" any Russian missile if development of the medium-range system continued.
The Russian foreign ministry reacted by saying Hutchison's comments were dangerous.
Hutchison later clarified in a tweet that she was not talking about a preemptive strike against Russia.
U.S. STARTING MISSILE RESEARCH
The top U.S. general in Europe said the United States needed to make strong statements on Russia's violation.
"We also will take the steps necessary to ensure that we don't have any gaps in a credible defence and deterrence posture," General Curtis Scaparrotti, head of U.S. European Command and NATO's supreme allied commander, told reporters during the NATO meeting.
A recent U.S. State Department report found Russia had violated obligations "not to possess, produce, or flight-test" a ground-launched cruise missile with a range capability of 500 km to 5,500 km (310-3,420 miles), "or to possess or produce launchers of such missiles".
Earlier this year, the U.S. military said in a new national defence strategy that countering Russia, along with China, would be a priority. The move reflects shifting U.S. priorities after more than a decade and a half of focusing on the fight against Islamist militants.
The Pentagon's nuclear policy document released in February said that in response to Russia's violation, the United States would start "commencing INF Treaty-compliant research and development by reviewing military concepts and options for conventional, ground-launched, intermediate-range missile systems"