Arbaeen marks the end of the 40-day mourning period for the killing of Imam Hussein, grandson of the Prophet Mohammed, by the forces of the caliph Yazid in 680 AD.

The annual pilgrimage usually sees millions of worshippers, mostly Iraqis and Iranians, converge on the central city of Karbala on foot.

Some 14 million attended in 2019, according to official figures, a third of them foreigners who came mostly from Iran, the Gulf, Pakistan and Lebanon.

Last year, Baghdad limited the number of foreigners to 1,500 per country because of the risk of Covid-19 infection.

This year, "30,000 pilgrims from the Islamic republic of Iran", Iraq's Shiite-majority neighbour, will be allowed to attend, according to a decision by Iraq's health and security committee.

There will be a quota of "10,000 pilgrims from Gulf countries, Arab countries and the rest of the world", it added.

The committee, presided over by Prime Minister Mustafa al-Kadhemi, said foreign pilgrims would be allowed to arrive only by air.

Arbaeen is marked 40 days after the Shiite commemoration of Ashura.

Millions of pilgrims thronged Karbala last month for that event, ignoring Covid-19 fears.

Health authorities have expressed alarm at similar gatherings during the pandemic.

"We have warned the Iraqi health ministry against all types of religious tourism," the World Health Organization's representative for Iraq Ahmed Zouiten told AFP, expressing fear that such gatherings could become super-spreader events.

Iraq, a country of around 40 million, has officially recorded almost two million Covid-19 cases and more than 21,000 deaths since the start of its outbreak.

Vaccination rates are low and measures such as social distancing and mask-wearing are widely ignored.