YENAGOA, Nigeria (Reuters) - Four Britons have been kidnapped in Nigeria’s southern Delta state, a police official said on Wednesday.

The police are attempting to rescue the four people, who were taken by unidentified gunmen on Oct. 13, said Andrew Aniamaka, a spokesman for Delta state police.

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.

“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 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.

There was an increase in crime in the southern region last year that coincided with a series of attacks on energy facilities. However, there have been no militant attacks on energy installations so far this year.