French authorities admit they do not know who is responsible for a series of unmanned drone flights at major sites in Paris.

A view shows The Invalides and  the Arc de Triomphe in Paris

Unidentified drones have been spotted in the skies above Paris for a second night as authorities seek to determine whether the aircraft pose a security threat.

Police say at least five drones were seen at locations including the US Embassy, the Eiffel Tower and major thoroughfares in central Paris, as well as several of the city gateways.

A police source said the unmanned aircraft were reported by witnesses and security forces.

A view shows the illuminated Eiffel Tower and La Defense business district in Paris

The source added: “Is it a game? Scouting for future operations? The investigation will show us.”

On Tuesday, police said unmanned aircraft were spotted at the same locations, leading a security source to admit “we don’t know” who is responsible.

The sightings are the latest in a series of unexplained drone flights in the French capital and elsewhere across the country.

Authorities were first alerted to mystery drone flights in October, when state-run power company EDF filed a complaint with police.

The company had detected tiny unmanned planes buzzing around the skies above seven atomic plants.

The sightings continued into November, and altogether some 20 flyovers took place near nuclear plants.

In January, a pilotless aircraft also flew over the presidential palace in Paris.

The mysterious flights come at a time when security measures have been stepped up in France in the wake of January’s Charlie Hebdo attacks.

In France it is illegal to fly small civilian drones over sensitive areas like nuclear facilities.

Sensitive areas are protected by a no-fly zone that spans a 1.6-mile radius to a height of 1,000 metres.