Indian authorities have been urged to abandon their efforts to recover the body of an American man killed while trying to preach Christianity to the isolated residents of a remote island.

Police in the Andaman and Nicobar Islands, an Indian territory in the Bay of Bengal, have made two boat trips to the area near North Sentinel Island since the missionary John Allen Chau was killed 11 days ago.

Chau, 26, was last seen by fishermen leaving for the island on the morning of 16 November after writing in his diary that he wanted to “declare Jesus” to its residents, members of a virtually “uncontacted” tribe thought to be at least 30,000 years old and known to aggressively resist outsiders.

Fishermen, who Chau allegedly paid 25,000 rupees (£277) to smuggle him to the island, say they saw what looked like his body being buried in the sand the next morning.

Police say they are consulting experts to decide whether it is feasible to retrieve Chau’s body, and will not provoke a confrontation with the Sentinelese, whose island is off limits to visitors without permission.

Survival International, a group which advocates for the rights of tribal peoples, has called on Indian authorities to abandon any recovery efforts, which it said would be “incredibly dangerous” for both sides.

“The risk of a deadly epidemic of flu, measles or other outside disease is very real, and increases with every such contact,” its director, Stephen Corry, said in a statement. “Mr Chau’s body should be left alone, as should the Sentinelese.”

A group of Indian anthropologists, authors and activists issued a similar statement on Monday. “The rights and the desires of the Sentinelese need to be respected and nothing is to be achieved by escalating the conflict and tension, and worse, to creating a situation where more harm is caused,” they said.

Dependra Pathak, the director of police in the Andamans, said a crime had taken place and police had an obligation to investigate – which could involve further surveys of the island to collect evidence to enable the issuing of a death certificate for Chau. “Based on requirements, further reconnaissance will be carried out,” he said on Tuesday.

Facebook Twitter Pinterest John Chau’s Instagram account described him as a ‘wilderness emergency medical technician’. Photograph: Reuters

A team of anthropologists are en route to the Andamans and will help police understand how they should proceed. “We have said from the beginning, we do not want a confrontation,” Pathak said. “As we draw up our strategy, and even to undertake a reconnaissance trip, we are in constant touch with academics and tribal welfare officials who have a better idea on [the tribe].”

He said police were trying to reconstruct the events that may have led to Chau’s death. According to his diary, he landed near the island on 15 November at about 4.30am, entering a cove on the western side and settling in the shallows.

After about four hours, when no Sentinelese tribespeople had appeared, Chau ventured on to the beach where, he wrote, he was confronted by several armed people with yellow paste smeared on their faces.

They chased him away and fired at him – piercing his waterproof bible – but Chau returned at about 2pm the same day and spotted a man he described wearing a crown of flowers and adopting a “leadership stance”.

Facebook Twitter Pinterest A Sentinelese tribesman photographed in 2004. Photograph: AFP/Getty Images

Chau dropped some fish, which he said the Sentinelese picked up. He tried to communicate with the group using a southern African language, but was greeted with silence and laughter.

That night, back on the boat, as he prepared to return the next morning, Chau wrote to his parents: “You guys might think I’m crazy in all this, but I think it’s worth it to declare Jesus to these people.

“Please do not be angry at them or at God if I get killed. Rather, please live your lives in obedience to whatever he has called you to and I’ll see you again when you pass through the veil.”