Rescuers were today searching for a British couple feared to have been kidnapped by pirates off the east coast of Africa.

Paul and Rachel Chandler, who were sailing from the Seychelles to Tanzania, have not been heard from since Friday, when the emergency beacon on their yacht, the Lynn Rival, was activated.

The British maritime and coastguard agency confirmed the beacon had been activated at 11pm on 23 October.

Officials said the Seychelles authorities were carrying out a search and rescue operation in the area where the boat was last known to have been. It is understood pirate activity was reported in the region earlier that day.

Reuters reported that the yacht had been seized by pirates and quoted one kidnapper, called Hassan, as saying: "The British couple are in our hands now. We captured them as they were touring in the Indian Ocean." He said the captives were healthy and ransom demands would follow.

The kidnap claim was echoed by Abdi Nor Osman, a self-proclaimed member of a pirate gang and resident of the coastal town of Haradhere, which is known to be a hotbed of pirate activity.

Osman said that he had spoken to a member of the gang that claimed to have captured the boat, adding that eight pirates were aboard the yacht, which was towing two skiffs. He said the yacht was expected into port at Haradhere today.

A Foreign Office spokeswoman said the reports were being investigated and that officials were in touch with relatives of the Chandlers, who are from Tunbridge Wells, in Kent.

She said it was possible that the yacht had simply strayed out of contact, adding that there was no indication of pirate involvement other than that reported in the media.

Combined Taskforce 151, an international naval response set up to combat Somali pirate attacks, is involved in the search, along with Nato and EU counter-piracy teams.

A spokesman for the EU naval force said: "Our ships will be scouring the seas on their normal passage and monitoring the airwaves."

He said the British couple's yacht had gone missing in "fairly dangerous waters" but there had been no confirmation that it had been hijacked by pirates.

A Royal Navy spokesman said HMS Cumberland was one of the ships involved in the piracy crackdown. "We are doing everything we can to assist the Foreign Office in investigating this," he added.

The couple's niece said that although the family had been alerted to the alarm signal on Friday, they had initially thought the situation was not serious.

"We were just waiting for them to come into docks because it tended to be the case that, out in the middle of the Indian Ocean, it was quite difficult to get hold of them and it was expected that they would dock at a little island," Leah Mickleborough told BBC Radio 5 Live.

"You never believe it's going to be one of those things that happens to your family. All of us as a family are extremely upset by what has happened.

"We are extremely distressed and it's such an emotional thing and such a horrible thing to be experiencing. I just hope they're OK. We all hope they are and that it can be resolved easily."

Mickleborough said her uncle and aunt were passionate about sailing but were aware of its perils.

"This is their life, really," she said. "They do sailing, they live for this. [But] they are not naive. They are very experienced in these things.

"They are not the sort of people who would put themselves deliberately in danger."

A British coastguard spokesman said: "They had left the Seychelles on 22 October and were going on a 150 nautical mile passage south-west to the Amirante Islands, en route to Tanzania.

"It would appear from the activation of the emergency position indicating radio beacon (EPIRB) that something has happened.

"We were aware that the EPIRB had gone off, talked to the Seychelles, asked if they were aware of it … they were, and have been searching by air and sea."

A spokesman for the Seychelles coastguard said they did not have any information about the couple.

"We did have a distress signal from them on 23 October. There have been reports that they were hijacked by pirates, but no one can prove that," he said.

"Our ship is in the area, and we are still keeping a lookout. We don't know whether they were continuing with their route. They don't have a satellite phone, so we cannot get hold of them."

The route would have taken the Chandlers, aged 58 and 55, near Somali waters notorious for pirate attacks on ships and smaller boats.

The last message on the couple's travel blog was posted on Friday morning and read: "Please ring Sarah." It is thought the message refers to Rachel's sister.

Earlier, the Chandlers – who have been sailing around the world for several years after selling up in the UK – wrote of "the Somali pirate problem" that had delayed other voyages to Tanzania.

Enthusiasts on a yachting forum questioned the wisdom of sailing in the area. One post said: "I hope the people in question are well. However, I do wonder why anyone would sail through these areas. The dangers are very well known and very well reported."

There have been a series of pirate attacks off Somalia in the last few years, with vessels from supertankers to cruise ships being targeted. Towards the end of last year, the situation became so bad that Maersk, the world's largest shipping company, announced that its oil tankers would be rerouted via the Cape of Good Hope.

As the presence of foreign navies in the Gulf of Aden and the Indian Ocean off Somalia's coast has increased, pirate gangs have been operating further south towards the Mozambique Channel.

The waters around the Seychelles are currently attractive to hijackers because of a return to calm seas following the monsoon season.

Launching high-speed skiffs from previously captured "mother ships", gunmen have seized vessels from Spain, Singapore and China in the region this month.

Andrew Mwangura, head of the East African Seafarers' Assistance Programme in Mombasa, said that while the Chandlers' route from the Seychelles to Tanzania was not usually used by cargo ships, the most common target for pirates, it was well within attacking range.

"With sea conditions so good the area is very favourable to pirates right now. Nowhere off the coast of east Africa is safe," he said.

If the yacht has been captured by pirates, it is most likely to be heading to Haradheere or Hobyo, both pirate havens on the Somali coast.

Graeme Gibbon-Brooks, the managing director of Dryad Maritime Intelligence Service, based in Southampton, said that, while there was no confirmation pirates had taken the missing boat, "it seems likely on the balance of probabilities".

"If this follows the normal pattern, the boat will be anchored off Somalia and the crew people taken ashore," he said.

"The question in this case is who will receive the ransom demand. The owner is on board and these are not very wealthy people, so we don't know how this will play out."