Russia has vetoed a resolution at the UN Security Council calling on all countries to prevent an arms race in outer space.
The draft resolution, put forward by the US and Japan, sought to reaffirm a principle already set out in the 1967 Outer Space Treaty.
The US has warned that Russia is believed to be developing a space-based, anti-satellite nuclear weapon.
Russia said it was “firmly committed” to the existing treaty.
The draft, put forward on Wednesday, called on “all States, in particular those with major space capabilities, to contribute actively to the objective of the peaceful use of outer space and of the prevention of an arms race in outer space.”
It also called on countries to uphold the Outer Space Treaty, under which all parties agreed “not to place in orbit around the Earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction”.
Of the council’s 15 members, 13 voted in favour, while Russia - one of five permanent members with a veto - voted against and China abstained.
Linda Thomas-Greenfield, the US ambassador to the UN, called the move “baffling”.
“Russia has vetoed a straightforward resolution that affirms a legally binding obligation,” she said. "President Putin himself has said publicly that Russia has no intention of deploying nuclear weapons in space.
“So today’s veto begs the question, why? Why, if you are following the rules, would you not support a resolution that reaffirms them? What could you possibly be hiding?”
In February, White House spokesperson John Kirby said Russia was developing a “troubling” new anti-satellite weapon, though added that the weapon was not yet operational.
The weapon was space-based and armed with a nuclear weapon to target satellites, the BBC’s US partner CBS News reported.
Russian President Vladimir Putin said in response that Moscow was “categorically against” the use of nuclear weapons in space.
More than any of its potential global adversaries, the US depends on satellite communications for everything from military operations and surveillance to civilian uses like GPS systems and financial transactions.
On Wednesday, US national security adviser Jake Sullivan reiterated that the US assessed that “Russia is developing a new satellite carrying a nuclear device”.
Details of the intelligence behind the claim have not been made public.
Russian Foreign Minister Sergei Lavrov said: “Russia is firmly committed to its international legal obligations, including 1967 The Outer Space Treaty.”
Russia’s envoy to the UN, Vassily Nebenzia, described the US-Japanese resolution as a “cynical ploy” with “hidden motives”.
The UN Security Council is made up of five permeant members - the US, UK, France, China, and Russia - each of which has a veto, and 10 seats that rotate between the other UN member states.