Bangladesh began transferring hundreds of Rohingya refugees on Thursday to a low-lying island in an area prone to cyclones and floods, with rights groups alleging people were being coerced into leaving.

Almost a million Rohingya -- most of whom fled a military offensive in neighbouring Myanmar in 2017 -- live in a vast network of squalid camps in south-eastern Bangladesh. With many refusing to return, and with violent drug gangs and extremists active on the sites, the Bangladeshi government has grown increasingly impatient to clear out the camps.

On Thursday more than 20 buses carrying almost a thousand people left the camps in the Cox's Bazar region, headed for the port city of Chittagong, said Anwar Hossain, regional police chief.

"Twenty buses left in two shifts. There were 423 people in the first 10 buses and 499 in the second 10 buses," he told AFP.

From Chittagong the refugees were due to be taken by military landing craft to the island of Bhashan Char, officials said.

The island, measuring 13,000 acres (52 square kilometres), is one of several silty strips to have surfaced in the Bay of Bengal in recent decades. The Bangladesh Navy has built shelters there for at least 100,000 Rohingya refugees as well as a nine-foot (three-metre) flood embankment.

But locals say high tides flooded the island a few years ago and that cyclones, a regular occurrence in the region, can cause storm surges of four or five metres.

'Smashed teeth'

Police said more buses would leave later on Thursday, with officials saying earlier they planned to transfer a total of 2,500 people in a first phase.

But rights groups including Human Rights Watch and Amnesty International alleged that some of the refugees had been coerced into going.

This was borne out by some family members that AFP spoke to on Thursday. "They beat my son mercilessly and even smashed his teeth so that he agreed to go to the island," said Sufia Khatun, 60, who came to see off her son and five other relatives. "I have come here to see him and his family probably for the last time," she told AFP in tears.

Hafez Ahmed, 17, came to say goodbye to his brother and his family. "My brother has been missing for the last two days. We later learnt that he is now here (transit camp), from where he will be taken to the island. He is not going willingly," Ahmed told AFP.

The United Nations office in Bangladesh issued a terse statement on Thursday saying it was "not involved" and had "limited information". It said the UN had not been allowed independently to assess the "safety, feasibility and sustainability" of the island as a place to live.

It said the refugees "must be able to make a free and informed decision about relocating" and that, once there, they should have access to education and health care -- and be able to leave if they wish.

'Better facilities'

But Bangladesh's Foreign Minister A.K. Abdul Momen called the rights groups' claims "a damn lie", and said the facilities on the island were "much better" than in the camps. "Bangladesh government has decided to take around 23,000 families to Bhashan Char voluntarily," he told AFP. "The current camps are very congested... They are going voluntarily."

Several local rights activists said some families have agreed to move to the island because of the prevailing law and order situation in the camps. At least seven Rohingya were killed and many houses were torched in recent months in attacks by suspected Rohingya extremist groups.

Since May the island has been home to 306 Rohingya refugees who were intercepted from boats on treacherous sea routes to Malaysia and Indonesia.