World Cup host Qatar is only just beginning to understand that it has a "huge problem" with the "forced labour" of South Asian migrants, the publishers of the first Global Slavery Index said on Friday.

Qatar is under mounting pressure to end the exploitation of migrant workers as it embarks on a multi-billion-dollar construction programme for the 2022 football World Cup.

The Australia-based Walk Free Foundation, which launched its inaugural index this week in London, said the spotlight would remain on the Gulf emirate as the tournament gets closer.

The report released this week -- a first attempt to measure the issue worldwide -- estimated that there were between 4,000 and 4,400 people in what it terms modern-day slavery in Qatar, out of a total resident population of two million.

"Qatar has millions of migrants, who come from South Asia predominantly, working in the country and then has a huge problem with those migrants, many of them being subject to forced labour," the foundation's chief executive Nick Grono told AFP.

"In our index we're trying to capture it but it's an open issue about whether we've effectively captured the scale of that particular problem. But we're very keen to work with authorities to draw their attention to this.

Grono said that as the tournament gets closer, the issue was "not going to go away".

"The Qatari authorities, I think, are just beginning to understand that this is an issue that needs to be addressed and we hope that with our index we start providing more data and more information to assist the policy response there.

"I hope we'll keep the spotlight on Qatar and all the other countries that have a big issue with modern slavery."

The foundation's definition includes slavery itself, forced labour, human trafficking and slavery-like practices such as debt bondage, forced marriage and the sale or exploitation of children.

Mauritania, Haiti and Pakistan topped the index, which ranked Qatar 96th out of 162 countries by prevalence of modern-day slavery.

The emirate was ranked ninth out of 19 countries in the Middle East and North Africa, with the regional list topped by Sudan, Libya, and Saudi Arabia.

The report said trafficking of migrants from sub-Saharan African and Asia was widespread in the region.

"Not all migrants are enslaved, but some will find themselves in situations that can foster enslavement," it said.

A report last month in British newspaper The Guardian said dozens of migrants from Nepal have died working in Qatar in recent weeks.

Campaigners say Qatar has anti-labour laws which prohibit migrant labourers from unionising or striking, while employers routinely confiscate passports, fail to pay wages and exert huge control over their subordinates.

World football's governing body FIFA has said it cannot interfere on labour rights, but admitted it cannot ignore them.

Qatar repeatedly rejects claims over slavery-style conditions on construction sites in the emirate -- the world's wealthiest nation per capita.