British Christian aid worker Ian Squire has been killed three weeks after being abducted by suspected militants in southern Nigeria.

Mr Squire was one of four British citizens kidnapped while working for a medical charity in Niger Delta.

The three other hostages, Alanna Carson and David and Shirley Donovan, were released after negotiations by British and Nigerian officials, Britain's Foreign Office has confirmed.

The four Britons were taken hostage on October 13 in Nigeria's oil-rich Delta region, where kidnappings are common.

Dr and Mrs Donovan had lived in Nigeria for the past 14 years and reportedly ran Christian charity New Foundations, which supplies aid to local villages.

According to the BBC, Mr Squire was an optometrist who was in Nigeria to carry out hundreds of cataract operations with New Foundations, assisted by fellow optometrist Ms Carson.

Mr Squire founded the charity Mission for Vision in 2003 and regularly travelled to remote regions in the developing world to carry out clinics and provide glasses, medication and cataract operations.

The Foreign Office said it was unable to give further details about the kidnapping because of an ongoing investigation by the Nigerian authorities.

"We are supporting the families of four British people who were abducted on October 13 in Nigeria, one of whom was tragically killed," the statement said.

"This has clearly been a traumatic time for all concerned, and our staff will continue to do all we can to support the families."

A statement from the families of Mr Squire and the other hostages said the last three weeks had been traumatic for the relatives and friends of those involved.

"We are delighted and relieved that Alanna, David and Shirley have returned home safely," the statement said.

"Our thoughts are now with the family and friends of Ian as we come to terms with his sad death."

ABC/Reuters