The Archbishop of Canterbury has joined a public outpouring of sympathy after a British missionary died in Nigeria.

Ian Squire, 56, was one of four aid workers kidnapped in the southern Delta state on October 13. Alongside David Donovan, a GP from Cambridge, Donovan's wife, Shirley and Alanna Carson, an optometrist from Leven, Fife, Squire, an optician, was working with the Christian health charity New Foundations.

The circumstances around Squire's death are unclear. A Foreign Office spokesman said he was 'tragically killed' while the others returned safely to the UK.

'This has clearly been a traumatic time for all concerned, and our staff will continue to do all we can to support the families. We are grateful to the Nigerian authorities, and are unable to comment given the ongoing nature of their investigations,' a statement read.

But reports from local media in Nigeria suggest Squire died of an asthma attack.

A security source told Punch, a Nigerian newspaper: 'Unfortunately, one of the male victims who is asthmatic and diabetic, died while in captivity as a result of complications from his ailments.' It is thought after Squire's death, the kidnappers panicked and dropped the other three hostages off in a village in Bayelsa state, near the area where they had originally been taken.

Justin Welby said his 'thoughts and prayers' were with Squire's family.

'We pray for the areas in the Niger Delta area of Nigeria where there are high levels of crime and tension – for peace, restoration and reconciliation and God's kingdom to come in those communities and across all of Africa,' he wrote on his Facebook.

In a statement following their release, the families of the hostages said: 'Alanna, Ian, David and Shirley were kidnapped in Nigeria some three weeks ago.

'We are grateful for the support received by the British High Commission, and help from the Nigerian authorities in negotiating their release. We are delighted and relieved that Alanna, David and Shirley have returned home safely. Our thoughts are now with the family and friends of Ian as we come to terms with his sad death.

'This has been a traumatic time for our loved ones who were kidnapped and for their families and friends here in the UK. We would therefore ask that the media respect our privacy as we come to terms with the news. We will not be making any further comment.'