Russia has said it is suspending inspections of its nuclear weapons by the US under a strategic arms reduction treaty with Washington, pointing to western sanctions and coronavirus infections.
The Russian Foreign Ministry said on Monday that sites which are subject to inspections under the New START treaty will be "temporarily" exempt from such scrutiny.
The announcement comes as ties between Russia and the US unravel over Moscow's intervention in Ukraine and resulting western sanctions.
New START is the last remaining arms reduction pact between the former Cold War rivals and caps the number of nuclear warheads that can be deployed by Moscow and Washington at 1,550.
"Russia is now forced to resort to this measure as a result of Washington's persistent desire to implicitly achieve a restart of inspections on conditions that do not take into account existing realities," the ministry said.
Moscow also accused Washington of seeking "to create unilateral advantages" and of depriving Russia of "the right to carry out inspections on American soil".
The ministry indicated it had become difficult for Moscow to carry out inspections on US territory owing to western sanctions, including visa restrictions and the closure of airspace for Russian planes.
Moscow also pointed to a new surge in coronavirus cases in the US.
"We believe that in the current circumstances, the parties should abandon patently counterproductive attempts to artificially speed up the resumption of START inspection activities and focus on a thorough study of all existing problems in this area," the foreign ministry said.
Last year, the US and Russia extended New START by the maximum allowed time of five years.
Moscow's announcement came after US President Joe Biden called on Russia and China to demonstrate their commitment to limiting nuclear arms.
Russia should demonstrate its willingness to renew the nuclear arms reduction pact when it expires in 2026, Mr Biden said.
A US State Department spokesman said that Washington is committed to the New START treaty, "but we keep discussions between the parties concerning treaty implementation confidential".
"The principles of reciprocity, mutual predictability and mutual stability will continue to guide the US approach to implementation of the New START Treaty," the official said.