A mystery Russian satellite’s “very abnormal behaviour” has sparked alarm in the US government.

State Department official Yleem Poblete told a conference in Switzerland, the US was unable to identify it or its purpose.

"We don't know for certain what it is and there is no way to verify it," the BBC reported her as saying.

Ms Poblete, Assistant Secretary of State for Arms Control, Verification, and Compliance refused to rule out that the satellite launched last October – could be a space weapon.

"[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 Geneva.

US State Department official Yleem Poblete. (twitter)

"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.

But Alexander Deyneko, a senior Russian diplomat, told the Reuters news agency her comments were "the same unfounded, slanderous accusations based on suspicions, on suppositions and so on".

Earlier this year, US President Donald Trump ordered the creation of a US space force.