WASHINGTON (Reuters) – The United States and Japan on Monday proposed a U.N. Security Council resolution stressing that nations should comply with a treaty that bars putting nuclear weapons in space, a message that appeared aimed at Russia.

Washington believes Moscow is developing a space-based anti-satellite nuclear weapon whose detonation could cause havoc by disrupting everything from military communications to phone-based ride services, a source familiar with the matter has said.

Russia, a party to the 1967 Outer Space Treaty that bars putting “in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction,” has previously said it opposes deploying nuclear weapons ins space.

Russia’s defense minister has also denied it is developing such a weapon. Deploying a nuclear weapon in orbit is barred by the treaty; developing one, however, is not prohibited.

In their resolution seen by Reuters, the United States, the only nation to use a nuclear weapon in war, and Japan, the only nation attacked with one, urged countries bound by the treaty not to place such weapons in space and also not to develop them.

Reports about possible Russian development emerged after a Republican lawmaker on Feb. 14 issued a cryptic statement warning of a “serious national security threat.”

The clearest public sign Washington thinks Moscow is working on such a weapon was a White House spokesman’s Feb. 15 comment that the lawmaker’s letter was related to a space-based anti-satellite weapon that Russia was developing but had not deployed, and that would violate the Outer Space Treaty.