A blue and white spiral spotted in the skies above numerous European countries was likely created by frozen fuel from a SpaceX rocket, according to weather forecasters.

The UK's Met Office said it had received many reports of an "illuminated swirl" in the British sky on Monday evening that was likely caused by a Falcon 9 rocket that had earlier launched from Cape Canaveral in Florida.

"The rocket's frozen exhaust plume appears to be spinning in the atmosphere and reflecting the sunlight, causing it to appear as a spiral in the sky," the Met Office explained in a post on X late Monday.

The Meteo weather forecaster in France's central Loire Valley area said it had also received many images sent in by keen skygazers.

"Don't panic! No, it wasn't a UFO but the degassing of a SpaceX Falcon 9 rocket," the forecaster said on X.

The swirls were caused the launch of SpaceX's NROL-69 mission, it added. 

The classified mission was conducted for the National Reconnaissance Office (NRO), which manages the US military's spy satellites.

"The Falcon 9 reusable rocket booster returned safely to Landing Zone 1 after delivering the national security payload to orbit," the office said in a statement.  

The spiral was also visible in Sweden, according to an image from a photographer in the country shared with AFP.

Social media users in other countries such as Croatia, Poland and Hungary also posted images of the phenomenon.

A similar spiral was spotted above New Zealand in 2022, also from the workhorse Falcon 9 rocket, according to reports at the time.

Billionaire Elon Musk's SpaceX has taken a "fail fast, learn fast" approach that has helped it become the world's dominant launch services provider.

The company's massive Starship rocket exploded during its latest test flight earlier this month, with social media footage showing red-hot debris raining down over the Bahamas.

Musk's status as one of President Donald Trump's closest advisors, and his influence over federal regulators, have raised concerns about potential conflicts of interest.

Eric Lagadec, an astrophysicist at the Cote d'Azur Observatory in France, reposted a video on the social media platform Bluesky that was apparently taken in Poland of the spiral rising in the sky.

Along with a gif of Musk dancing, Lagadec said: "I have a feeling this is a guy who likes to mess things up on Earth and above!"