The regional health board testing centre in Bovenkarspel in Noord Holland has been hit by an explosion, but there were no injuries, police say.

The explosion happened shortly before 7am and the test centre appears to have been the target, local media report. One security guard was on duty at the time.

Broadcaster NOS said a metal pipe exploded outside the building on the Middenweg, but this has not been confirmed. The area has been sealed off while explosives experts check to make sure the location is safe.

The number of positive test results in the Bovenkarspel region has been rising sharply for several days, particularly in nearby Hoorn. Hoorn is now fourth in the list of towns and cities with the most infections per head of the population.

This is not the first time a test centre has been the target of an attack. Before Christmas, vandals set fire to the test centre on the former island of Urk and in January, efforts were made to set fire to the Hilversum centre.

Windows were also smashed at test centres in Amsterdam and in Urmond in Limburg in December.