Fears of an outbreak of the deadly MERS virus in Saudi Arabia and construction in the holy city of Mecca have forced cuts in the numbers of pilgrims permitted to perform this year's hajj.

Millions of Muslims during the annual pilgrimage head to Mecca and Medina, Islam's two holiest sites, providing a possible means for MERS to spread around the globe as pilgrims who may become infected return to their home countries.

Fearful of such a scenario, the authorities have reduced by half the number of pilgrims coming from within Saudi Arabia, and by about 20 percent those from abroad.

"This is an exceptional and temporary decision," Hajj Minister Bandar Hajjar announced last month.

MERS, short for Middle East Respiratory Syndrome coronavirus which can lead to acute pneumonis and renal failure, claimed its first victim in Saudi Arabia in June 2012.

Saudi Arabia, which has already recorded 69 MERS cases, 38 of whom have died, has urged the elderly and chronically ill, as well as children and pregnant women, not to perform the annual hajj that falls this year in October.

But the health ministry so far has not taken any special measures at airports to detect visitors who may be infected with MERS, deputy health minister Ziad Memish told AFP.

"We have not taken any precautionary measures at airports since the World Health Organisation has not recommended them," he said.

WHO health security chief Keiji Fukuda said the organisation would issue general guidelines aimed at minimising the risk of infections spreading.

"We do recognise that this is a risk for travellers and that there are certain steps that individual travellers and countries can take, for example for people who have serious medical conditions," Fukuda told reporters.

Since then, a total of 90 cases and 45 confirmed MERS deaths have been recorded worldwide, in countries including Jordan, Qatar and the United Arab Emirates, Tunisia, Britain, France and Italy.

But most cases have been reported in Saudi Arabia.

Experts are struggling to understand MERS, for which there is no vaccine.

MERS does not appear to spread easily but currently has an extremely high fatality rate of 55 percent. It is a cousin of SARS, which erupted in Asia in 2003 and infected 8,273 people, nine percent of whom died.

Like SARS, MERS is thought to have jumped from animals to humans, and shares the former's flu-like symptoms -- but differs by causing kidney failure.

The 2012 hajj drew 3.1 million people -- and this year's event likewise occurs in October, as the northern hemisphere slides into the season for coughs and sneezes.

-- 'We have to be worried' --

So far, MERS has essentially been found in nations with health services capable of tracing and tackling such diseases. But the hajj draws a broad spectrum of Muslims, including from poor countries which struggle to cope even with commonplace diseases.

The hajj has successfully ridden out two previous viral episodes in the past decade -- SARS in 2003 and H1N1 influenza in 2009.

The difference this time is that Saudi Arabia itself is the apparent incubator of MERS.

Leading virologist Laurent Kaiser of Geneva University Hospitals told AFP: "It's really a balance between too much precaution and no precaution. At this time, we have to be worried, we have to be careful."

So far no outbreaks of the virus have been reported in Mecca where millions of Muslim faithful have for months been performing the minor pilgrimage umrah that takes place all year round.

"Their numbers have reached five million since the beginning of the umrah season" 10 months ago, Mecca governor Prince Khaled al-Faisal said on Sunday.

"There are currently 400,000 pilgrims" in the kingdom's holiest shrines Mecca and Medina, Faisal added.

Aside from the virus fears, Saudi authorities have also cited construction work to expand the Grand Mosque in Mecca as reason to keep down the number of pilgrims being allowed to perform this year's hajj.

Hajjar said the expansion work would increase the area of the mosque by 400,000 square metres (4.3 million square feet), raising its capacity to accommodate 2.2 million people at the same time.

The mosque houses the Kaaba -- the cube-shaped structure towards which Muslims worldwide pray.

Hajj officials have also reported a decrease in the numbers of pilgrims performing umrah during the Muslim fasting month of Ramadan that began on July 10, compared with past years, due to the expansion work.

"Companies here have received permits for 500,000 pilgrims from all over the world for Ramadan, while this number was one million last year," a board member of the Mecca Chamber of Commerce, Saad al-Qurashi, told AFP.