The Nigerian activist Sunny Ofehe, kidnapped with three Dutch nationals on Sunday in Nigeria has been freed, Dutch media report.

The Dutch nationals are still being held and a €10m ransom demand has been made for their release, Nigerian police told RTL news on Tuesday morning.

Ofehe, an activist who lives in the Netherlands, and the other three were taken by armed militia after visiting a clinic in the Niger Delta.

Two of the other three are said to be Erhard Leffers and Marianne Vos, who work for a printing company in Dedemsvaart and with Ofehe produce a magazine about the Niger Delta. The third is documentary maker Jan Dries Groenendijk.

A Nigerian journalist who was on the boat with the four said the militants screamed at everyone to lie on the deck. ‘They collected our things, mobile phones and cameras,’ he said. Someone then shouted ‘take the white people’, he told news website Sahara Reporters.

It is not clear who took the four, but Nigerian police say it was not the Muslim extremist group Boko Haram.