Three people have been killed and one seriously injured after becoming trapped in a manure silo on a farm in Friesland, website nu.nl reports.

A fourth man has been taken to hospital in Groningen in a very serious condition.

Nos television reports that one of the dead is the 26-year-old son of the farm owner who was due to take over the business shortly.

The four men were working on the silo when they became trapped although the circumstances of the accident are still unclear.

The silo was almost empty, and according to Frisian media reports, one man may have gone into clean the silo wearing a gasmask when he became unwell. The others then went in to try and help him.

A police spokesman told nu.nl he climbed to the top of the 10m high silo on arrival at the farm in Makkinga and saw four men lying in the manure. Rescue workers eventually broke open the side of the silo to reach the men.

Two air ambulances, firemen and a diving team were used in the attempt to rescue the men.