Police in Groningen have issued an amber alert for two children, aged 10 and eight, who are believed to have been taken by their father and could be in serious danger.

Jeffrey and Emma have been missing from near the village of Beerta, close to the German border since Saturday afternoon. They are thought to have been taken in a grey Toyota Avensis, registration 77-NLV-4. German police are also involved in the search.

On Sunday evening, the operation focused on the Carel Coenraad polder in Finsterwolde following several tip-offs, but no trace has yet been found of the children or their father, Klaas Bijl (67).

Police say they have “serious concerns” for the children’s safety and that finding them is the “highest priority”.

Jeffrey is 1.5 metres tall and was wearing a yellow shirt, black shoes and grey walking boots when he disappeared. Emma is 1.3 metres tall and was wearing pink trainers.

An older brother of the children told television show Hart van Nederland on Sunday evening that Bijl had been unable to accept the end of his marriage. “Since then he has gone crazy,” the brother said.