It's feared a mysterious Russian satellite could be the world's first deployment of a space weapon.
The satellite is - officially - a "space apparatus inspector", designed to inspect other satellites - but a top-ranking US diplomat has told the UN that it isn't acting like one.
"Its behaviour [is] inconsistent with anything seen before from on-orbit inspection or space situational awareness capabilities, including other Russian inspection satellite activities," US assistant secretary for arms contro, Yleem Poblete said at the UN Conference on Disarmament in Geneva, Switzerland.
"We don't know for certain what it is and there is no way to verify it. But Russian intentions with respect to this satellite are unclear and are obviously a very troubling development."
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It's not clear which satellite Ms Poblete was talking about - she doesn't mention it by name - but speculation has settled on a trio of satellites called Kosmos 2519, 2521 and 2523.
They were put into orbit about a year ago. Suggestions from the US they were for anything other than peaceful purposes was dismissed as "slanderous accusations based on suspicions, on suppositions and so on", a Russian delegate told media at the conference.
Both the US and Russia have made moves to strengthen their military positions - Russian President Vladimir Putin unveiling powerful new weapons back in June, and his US counterpart Donald Trump pushing for the militarisation of the final frontier in the form of a 'Space Force'.
"Our adversaries have transformed space into a warfighting domain already," US Vice President Mike Pence said last week. "The United States will not shrink from this challenge. Under President Trump's leadership, we will meet it head on to defend our nation."
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Russia has in the past said it was developing anti-satellite technology, for use in the event of a future war. But it's also promoted a proposed treaty - the Treaty on the Prevention of the Placement of Weapons in Outer Space, the Threat or Use of Force against Outer Space Objects (PPWT) - that would ban putting weapons in space and attacking satellites.
Ms Poblete said Russia couldn't be trusted either way.
"In view of Russian noncompliance with its arms control commitments... is this inconsistent behaviour I have noted what we might expect in the future?"
The Outer Space Treaty, signed by the US, UK, and USSR in 1967, bans stationing weapons of mass destruction in space, or using any weapons on the moon. Missiles launched from Earth that travel through space aren't prohibited.
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