By BBC News

The Bangladeshi government has started its first census of unregistered members of the Rohingya ethnic group.

It is thought more than half a million live in the country without authorisation after fleeing persecution in Myanmar, also known as Burma.

Many Rohingya people are stateless, and they have been subject to people-trafficking in large numbers.

The census will last several weeks and organisers hope to find out more about Rohingyas’ socio-economic conditions.

Many are very poor. Tens of thousands are registered as living in two refugee camps in Bangladesh.

The census will be carried out in co-operation with the International Organization for Migration and will involve house visits. So far there have only been small-scale research projects and estimates of the numbers of unregistered Rohingyas there.

Rohingyas are a distinct Muslim ethnic group. In Myanmar, from which many have fled, Rohingyas’ rights are heavily restricted. Most are not recognised as Myanmar citizens.

In 2015 tens of thousands were stranded at sea after surrounding countries would not let them in.

Why did so many Rohingyas flee Myanmar?

While the Rohingyas say they are descendants of Arab traders who have been in the region for generations, Myanmar’s governments say they are not a genuine ethnic group but are actually Bengali migrants.

Successive Myanmar governments have been introducing policies to repress the Rohingya since the 1970s, according to Burmese Rohingya Organisation UK (Brouk).

They are denied basic services and their movements are severely restricted.

In June and October 2012 there were large scale attacks on Rohingyas following the gang rape of a Buddhist woman.

Who are the Rohingyas?

• Rohingyas are a distinct, Muslim ethnic group mainly living in Myanmar
• Thought to be descended from Muslim traders who settled there more than 1,000 years ago
• Also live in Bangladesh, Saudi Arabia and Pakistan
• In Myanmar, they are subjected to forced labour, have no land rights, and are heavily restricted
• In Bangladesh many are also desperately poor, with no documents or job prospects