Derk Bolt and Eugenio Follender went missing at the weekend in Catatumbo, a region where several armed groups operate, including the left-wing ELN.
They were searching for the mother of a Colombian child adopted in the Netherlands a few years ago.
The Colombian authorities tweeted a demand "for the immediate release of the two men."
Last year the ELN rebel group kidnapped a Spanish journalist and several Colombians in the same area. All were later released.
The Colombian army says it has sent specialist forces to the region to search for the Dutchmen.
