“We don’t know for certain what it is and there is no way to verify it,” said assistant secretary Yleem Poblete at a conference in Switzerland on 14 August.
She voiced fears that it was impossible to say if the object may be a weapon.
Russia has dismissed the comments as “unfounded, slanderous accusations based on suspicions”.
The satellite in question was launched in October last year.
“[The satellite’s] behaviour on-orbit was inconsistent with anything seen before from on-orbit inspection or space situational awareness capabilities, including other Russian inspection satellite activities,” Ms Poblete told the conference on disarmament in Switzerland.
“Russian intentions with respect to this satellite are unclear and are obviously a very troubling development,” she added, citing recent comments made by the commander of Russia’s Space Forces, who said adopting “new prototypes of weapons” was a key objective for the force.
Ms Poblete said that the US had “serious concerns” that Russia was developing anti-satellite weapons.
Alexander Deynko, a senior Russian diplomat, told the Reuters news agency that the comments were “the same unfounded, slanderous accusations based on suspicions, on suppositions and so on”.
He called on the US to contribute to a Russian-Chinese treaty that seeks to prevent an arms race in space.
‘Lasers or microwaves’
Space weapons may be designed to cause damage in more subtle ways than traditional weapons like guns, which could cause a lot of debris in orbit, explained Alexandra Stickings, a research analyst at the Royal United Services Institute.
“[Such weapons may include] lasers or microwave frequencies that could just stop [a satellite] working for a time, either disable it permanently without destroying it or disrupt it via jamming,” she said.
But it was difficult to know what technology is available because so much information on space-based capabilities is classified, she added.
She also said it would be very difficult to prove that any event causing interference in space was an intentional, hostile action by a specific nation state.
Ms Poblete’s comments were particularly interesting in light of President Donald Trump’s decision to launch a sixth branch of the US armed forces named Space Force, added Ms Stickings.
“The narrative coming from the US is, ‘space was really peaceful, now look at what the Russians and Chinese are doing’ – ignoring the fact that the US has developed its own capabilities.”
A spokesman for the UK’s Ministry of Defence said he could neither confirm nor deny any tracking of Russian satellites.
“There are a range of threats and hazards to all space capabilities in what is an increasingly contested domain,” he said.
“These include the development of counter-space weapons by a number of nations.
“The UK is working closely with international allies, including the US, to re-enforce responsible and safe behaviours in space and to build knowledge, understanding and resilience.”