A letter bomb has exploded at the offices of the Swiss nuclear energy association, injuring two people, police have said.

The letter exploded at the offices of Swissnuclear in the northern town of Olten, a police spokesman said. Both victims suffered superficial wounds.

“At about 8.15 am, while opening the letter, it exploded,” injuring two people, a spokesman for police in the canton of Solothurn told AFP.

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“One person was hurt in the arm,” he said, without giving details about the identities of the victims.

No-one has claimed responsibility for the attack.

Swissnuclear is the nuclear energy offshoot of the Swiss power industry association. It groups major power generating firms in Switzerland, according to the association’s website. Swissnuclear was not immediately available for comment.

The entrance of the office building, located in the centre of Olten, as well as a part of the street, were blocked off by police.

No damage was visible from the exterior of the building, which was surrounded by police cars.

Thursday’s incident occurred as around 30 Greenpeace protestors held a demonstration a few hundred metres away, outside the headquarters of Swiss power firm Alpiq.

But the group quickly suspended its demonstration following the blast, distancing itself from the letter bomb.

“We distance ourselves with the greatest firmness from this explosion.

Greenpeace has nothing to do with this attack,” Florian Kasser, who heads Greenpeace’s energy campaign in Switzerland, told AFP.

“The demonstration has now been suspended,” he added.

In a statement, Greenpeace said the demonstration was being held to demand Alpiq formally withdraw a request to build new nuclear power plants in Switzerland, which has suspended plans to replace its ageing reactors following Japan’s nuclear disaster.

In an interview with a Swiss newspaper, Heinz Karrer, who heads Swiss energy group Axpo, acknowledged that “at the moment, it is unthinkable to talk about another new nuclear power plant.”