U.S. officials are preparing to announce restrictions to Russian military flights over American territory under the Treaty on Open Skies in the latest sign of strain in U.S.-Russia relations, The Wall Street Journal reported.

The treaty, part of a suite of arms-control, transparency and confidence-building agreements that proliferated in the late stages of the Cold War, permits its 34 ratified member states to conduct observation flights over each other’s territory while capturing aerial imagery of military personnel and materiel.

Treaty-related tensions intensified over two days in August, when a Russian plane operating under Open Skies flew over U.S. cities including Washington as well as Bedminster, N.J., while President Trump was staying at the Trump National Golf Club in the town.

Those flights, however, aren’t the cause of the new restrictions, according to U.S. officials. Instead, they assert that Russia is in violation of the Open Skies treaty, because the Kremlin imposed restrictions on flights over Kaliningrad, Russia’s Baltic Sea exclave, which U.S. officials believe is host to a cache of sophisticated weaponry.