Down a narrow alleyway deep in the Jeddah slum of Karantina, three women from Sudan have set up stalls under colourful parasols, selling peanuts, hibiscus petals, dried pulses, baskets, frankincense, calabashes and sandalwood.

They laugh and gossip in the sunshine, swathed in bright printed cloth, while a scrawny black cat picks its way between piles of rubbish. But when approached by a stranger, they are cautious.

Jeddah has attracted outsiders for centuries, being the main port of arrival for Muslims making the haj pilgrimage to Mecca. It is this history that gives Karantina its name: older residents can remember when it was "quarantine" for pilgrims.

But the people who now live in this slum in the south of Saudi Arabia's second biggest city were drawn not only by religious devotion but also the top oil exporter's wealth. They live in a legal limbo, sometimes for generations.

"This is the forgotten area," said a bearded Sudanese man in his 40s. "Here are many illegal immigrants who don't have official papers. Government supervision is scarce."

Saudi Arabia's hundreds of thousands of illegal immigrants are not counted among the millions of expatriates who reside legally in the Arab kingdom, working as everything from maids to finance executives.

Instead they live on the margins, ineligible for government services and outside of the law, but often unofficially tolerated because of the expense and administrative obstacles in the way of expelling them.

In recent months, however, their status has caught the attention of Saudi media, who have been calling them "infiltrators" and warning readers of their supposed links to crime, disease and militancy.

"The infiltrators will carry with them all their social ills including security threats, criminal behaviour and ethical issues," wrote commentator Hamoud Abu Talib in an opinion piece in Okaz daily this month.

A spokesman for the Interior Ministry, Mansour al-Turki, said the media has exaggerated the number of crimes committed by illegal immigrants and added that Saudi citizens themselves contributed to the problem by using them for cheap labour.

Many illegal immigrants have now lived in Saudi Arabia for decades, having children and grandchildren who now live without nationality or residence papers, and prompting government officials to speak of a "humanitarian crisis".

Some risked a perilous journey through volatile Somalia and Yemen, others overstayed work visas or came to perform the annual haj and never went home.

In 2008 Saudi officials told American diplomats that around 10 per cent of pilgrims overstayed their visas each year, a US embassy cable released by WikiLeaks revealed. Last year more than 2 million people came on haj from overseas.

Last week local media reported police in Asir Province bordering Yemen as saying 1,470 illegal immigrants had been arrested in just two days.

"Dealing with these problems is not easy once they're in the country ... Sometimes you can't prove their nationality," Turki told Reuters. "You cannot send them back to Yemen. They will not take them."

Turki was not able to estimate the number of illegal immigrants in the country.

In Karantina, people sit out on doorsteps or high metal benches covered with strips of loose carpet, chatting with neighbours. Old sofas and armchairs sit propped against houses.

Rubbish carpets both sides of the road, and abandoned, broken-down cars and trucks gather dust, sinking onto flat tyres.

In a large open space near a highway where trucks park, old cars lie in deep puddles of water and mud, some submerged up to their axles, while wading birds strut between them on long legs.

One 15-year-old girl, speaking to Reuters as she watched a television soap opera in a room choked with wood smoke, said her family came from Chad before she was born.

The girl's family - mother, brother, father, his three other wives and their children - are among the luckier residents of Karantina, giving out food as charity to those in need.

"I'm happy. There is nothing more that I can ask for. My father provides me with everything. I have a television," she said.

Like the other residents approached by Reuters, the family members declined to give their names for fear of attracting the attention of the authorities and having to leave the country.

Immigrants who do not have a residency permit, known as an iqama, face restricted access to medical care and other services. Residents of Karantina said they have to pay more for treatment, which by law should only be provided to people who can show valid identification papers.

A small private school in Karantina teaches the Koran to local children, but it is unregulated. Prince Khaled al-Faisal, governor of Mecca Province, which includes Jeddah, said last year there were around 1 million illegal residents in the province's slums.

For the government, the presence of so many undocumented people has a big impact on its efforts to strengthen labour laws. The country is trying to encourage firms to hire more Saudi citizens, who now make up only about a tenth of private-sector employees.

This month the Labour Ministry said it would fine companies found employing illegal immigrants. It also offered incentives to firms that employed Palestinian and Burmese refugees with legitimate residency papers.

On Sunday, Prince Khaled said the government would give legal residency to 250,000 Burmese Muslims already living as refugees in Saudi Arabia.

As holders of the iqama they will be able to access government services more easily and work, drive and rent accommodation legally, in common with millions of other foreigners living in Saudi Arabia.

Even for those with residency, however, life as an immigrant in the kingdom can be tough.

"I never thought of returning to Burkina Faso. All my children were born here. Thank God, we live with the help of charitable people," said Sadiq Basheer al-Sadiq, who came to Saudi Arabia on haj by sea in 1970 and is now the patriarch of a family 60 strong.

He said he and all his family members, including 15 children by four wives, had legal residency.

They live in a small plot of land in the Ruweis district of central Jeddah, their open-air rooms covered by aluminium roofs and divided by wooden beams and cement bricks.

Sadiq, now 85, has retired after working as a construction worker in Mecca, and is now dependent on his children.

His son, Ibrahim, was born in Saudi Arabia but has no hopes of ever qualifying for citizenship of a country where local passport holders qualify for substantial state benefits. "We did not even try," he said.-Reuters