Police cordoned off a perimeter of 100 metres (328 ft) around the headquarters, according to the police web page. They said a "handful" of people were taken to hospital, including some police, after detecting an odour at the scene.

The Security Service declined to comment on the matter.

The cordon lasted for four hours before the local police lifted it and said the people taken to hospital were feeling well. A preliminary investigation will be initiated, a spokesperson told Reuters, adding no foul play was suspected.

Minister of Justice Gunnar Strommer told TT news agency that the government was following the situation and in contact with the Security Services.

Earlier on Friday, Swedish Prime Minister Ulf Kristersson held a joint news conference with Hungarian counterpart Viktor Orban as the Hungarian parliament is set to ratify Sweden's NATO application next week - the only remaining hurdle before Sweden can become member of the military alliance.

(Reporting by Johan Ahlander, Anna Ringstrom and Essi Lehto; Editing by Jan Harvey and Josie Kao)