A mysterious Russian satellite displaying "very abnormal behaviour" has raised alarm in the US, according to a State Department official.
"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 suspicious".
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.