Aliens, Russians, or Space Police propaganda? Mystery space object raises alarm in U.S.
"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.
She voiced fears that it was impossible to say if the object may be a Russian 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," 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.
"We are concerned with what appears to be very abnormal behavior by a declared 'space apparatus inspector.' We don’t know for certain what it is and there is no way to verify it."
Ms Poblete said that the U.S. had "serious concerns" that Russia was developing anti-satellite weapons.
Alexander Deyneko, 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 U.S. to contribute to a Russian-Chinese treaty that seeks to prevent an arms race in space. ■