The Open Skies Treaty which allows superpowers Russia and the United States to overfly each other to ensure neither is up to anything suspicious is in danger of collapse.
The post Cold War arm-control treaty allows declared, unarmed aerial surveillance flights over the entire territory of member nations. Its intention is to demonstrate that none of the 34 nations that are signatory to the agreement are engaged in any secret military build-up or expansion program.
But it seems Russia now regards some of its territory less open than others.
A Tu-160 bomber lands at Engels-2 airbase on August 7, 2008 in Engels, Russia. Photo / Getty Images
The Wall Street Journal reports the United States will restrict Russian military observation flights over some US territory after Moscow began blocking similar overflights of its increasingly heavily militarised Kaliningrad enclave on Europe's Baltic coast, Chechnya, South Ossetia and Moscow itself.
Earlier this week, the Chairman of the Pentagon's Joint Chiefs of Staff, General Joseph Dunford, told a US Senate Armed Services Committee that Washington wanted the Open Skies Treaty with Russia to remain in place.
He added there was little chance of this happening if Moscow continued flouting the agreement.
Overnight, Russian news agencies reported Russian Deputy Foreign Minister Sergei Ryabkov as saying Moscow was also unhappy at US compliance with the treaty - and would take action in response to any new restrictions.
"I have no doubt there will be a (Russian) response," Ryabkov reportedly said.
"But before announcing something on this, we have to analyse the situation with our military and look at how we'll respond to the Americans."
While the Open Skies Treaty allows aircraft to range a total of 5500km over a nation's air space, Moscow has imposed a new 500km limit for Kaliningrad. It takes a 1200km flight to cover the entirety of the fortress-enclave.
The region - bounded by Poland, Latvia, Lithuania, Estonia and Sweden - has been the scene of increasingly tense military stand-offs in recent years. A major military exercise earlier this month raised deep suspicions that Russia was exercising its forces to a far greater extent than it admitted.
A handout picture released by the British Ministry of Defence shows a Russian TU-160 Blackjack bomber with French Rafale aircraft in the air outside British airspace. Photo / MOD
Kaliningrad has in recent years seen a significant expansion of its military facilities, including the deployment of the most modern anti-aircraft systems and intermediate ballistic missiles capable of carrying both nuclear and conventional warheads.
Neighbouring NATO nations are wondering why.
Tensions were further inflamed by the "undiplomatic" overflight of the US cities of Washington and Bedminster in August by a Russian surveillance aircraft - while President Donald Trump was present.
Russia, in turn, argues Canada, Georgia, Turkey and the US have been placing limits on its flights. It cited the example of a flight over Alaska in July which was ordered to increase its altitude while flying over Alaska.
But Moscow itself has been the subject of new Russian restrictions, General Dunford said, with recently imposed limits on the heights aircraft could fly while undertaking their photographic runs.
