A Russian ambulance spotted in central Stockholm sparked far-fetched fears on Twitter that the city might be undergoing an invasion.

The ambulance, a good 865 kilometers from the Russian border, was first noticed by local Måns Jonasson when it stopped to help a sick man in Stockholm's Södermalm district.

To the great surprise of passers-by, the ambulance had Russian plates and inscriptions, and its Russian-speaking personnel wore uniforms with Russian script.

Swedes have a perennial fear of "Russian invasion," and the unexpected guest – although benevolent – played into their anxiety.

While some who responded to Jonasson's tweet guessed a film was being shot in the neighborhood, most claimed the vehicle was sent by the Russian embassy.

Stockholm police showed special interest in the case, but they too were stumped by the apparent detour, according to daily tabloid Aftonbladet.

"Nobody seems to know," said policeman Victor Adolphson.

The strange sighting was later explained by the head physician at the ambulance's private medical center in St. Petersburg.

The vehicle had been sent to fetch a Russian citizen injured during a work trip to the capital city, Mikhail Dzalagonia clarified.

The vehicle had stopped to assist the sick man after being flagged down by locals.

"Our doctors provided him first aid before the arrival of their Swedish colleagues," Dzalagonia told the Russian agency RIA Novosti.

Jonasson, the man who sparked the Twitter flurry, later sent a tweet to his Russian neighbors, absolving himself.

"I didn't think there was anything nefarious about a Russian ambulance in Stockholm, I was just curious as Itarget="_blank"'>