Four British missionaries have been kidnapped in Nigeria's southern Delta state, police revealed Wednesday.

The four Christians had been working to give out free medical supplies in a remote part of the country when they were taken by gunmen on October 13, a spokesman for Delta state police revealed on Wednesday.

'The abductors have not made any contact but we are doing our investigations to know the motive and have them rescued without jeopardising their lives,' said Andrew Aniamaka.

'Information available to us shows they are missionaries giving free medical services,' he said, adding that the British nationals had been working in a very rural area.

Kidnapping for ransom is a common problem in parts of Nigeria. A number of foreigners have, in the last few years, been kidnapped in the Niger Delta region, which holds most of the country's crude oil - the country's economic mainstay.

But Christians have been particularly targeted in the country, although the attacks have tended to be in the Muslim majority north, rather than the more Christian south of Nigeria. 

A major report last month warned the Nigerian government was not doing enough to protect remote Christian farmers from being targeted by Fulani herdsman.

Additional reporting from Reuters.