German authorities said on Friday that unidentified drones had been spotted flying over sensitive military and industrial sites.

Among the sites was the US air base at Ramstein in the western state of Rhineland-Palatinate.

What do we know about the sightings?

The sightings were first reported by Germany's Der Spiegel magazine.

It cited a report from German security authorities saying that several drone sightings were registered over the Ramstein base late on December 3 and 4.

Der Spiegel said that unidentified drones were also sighted over facilities belonging to German arms maker Rheinmetall.

Unidentified drones were also seen over the Brunsbüttel area of northern Schleswig-Holstein state in August.

Media reports said officials believed at the time that the drones were part of a Russian espionage campaign.

However, the ARD broadcaster reported on Friday that investigations into the sightings in Brunsbüttel have as of yet unearthed no indications of espionage.

'No impact' on facilities — US Air Force

A spokesperson for the US Air Force said that its facilities at the Ramstein air base were unaffected.

There were "no impacts to base residents, facilities, or assets," the spokesperson said.

"In concert with host nation authorities, we continue to monitor the airspace to ensure safety and security of the community."

Over the past few weeks, unidentified drones were also sighted in New Jersey and other parts of the northeastern United States.

On Thursday, the US federal authorities said in a report that there was no evidence that the sightings posed a national security threat and that some of the devices were "manned aircraft … operating lawfully."

In November, the US Air Force said that unidentified drones were spotted over three air bases in the United Kingdom.

sdi/rm (AFP, Reuters)