Four Dutch citizens suspected of dealing in deadly sarin nerve gas appeared before a judge on Tuesday, a justice ministry official said, with the case still cloaked in mystery.

The four suspects, two men aged 21 and 35 and two women, aged 33 and 52, face charges of "attempting to transfer, possession or use of a toxic substance, possibly sarin", Public Prosecutor spokeswoman Cindy Reijnders told AFP.

They were arrested in the southern Netherlands near the German-Belgian border on Saturday night after police got a tip-off of a possible sale of the deadly nerve gas.

Two of the suspects were taken into custody as they began digging in a forested area just outside the popular tourist city of Maastricht, allegedly looking for the sarin.

The two other suspects were arrested in Maastricht and the nearby town of Heerlen at the same time, Dutch police said on Monday.

Authorities cordoned off the area and started an intensive search using high-tech x-ray and infrared equipment. They also dug up an area of around 400 square metres (4,300 square feet), watched by dozens of curious onlookers.

At a press conference on Monday evening, justice and police officials revealed that they had been searching for sarin nerve gas, but stressed they had found nothing.

Officials said there was no danger to the public.

Reijnders declined to give the names of the suspects, who have been remanded into custody.

Three of the four suspects are Dutch nationals and the fourth, identified by Dutch media as "Islam A.", has dual Dutch-Turkish nationality.

"Because the suspects are being held under strict conditions, we cannot release more information about the investigation," Reijnders said.

Officials said the sale could not be linked to any act of terror or ideological motives and "seemed purely for financial gain".

Dutch popular broadsheet De Telegraaf reported Tuesday that a lovers' tiff may have played a role and that the target could have been the mistress of one of the suspects.

When asked about the report, Reijnders declined to comment.

Developed in 1938 in Germany as a pesticide, sarin is a deadly and volatile nerve agent that is colourless, odourless.

In high doses, sarin paralyses the muscles around the lungs and prevents chemicals from "switching off" the body's secretions, so victims suffocate or drown as their lungs fill with mucus and saliva.