Health authorities in Saudi Arabia are on top alert after the death of four pilgrims from swine flu, with fears this year’s Haj season becoming a breeding ground for the virus.

Health ministry spokesman Dr Khaled Marghlani told reporters that 16 other people had so far been diagnosed with the disease and that four remained in hospital in a critical condition.

Officials had attempted to prepare for any outbreaks by installing thermal imaging cameras at immigration checkpoints at airports, deploying 15,000 additional healthcare workers and ensuring that hundreds of extra beds were on hand.

In order for pilgrims to enter the kingdom, they will now require a certificate of vaccination before they can apply for a visa.

The Haj is one of the five pillars of Islam and is central to the nation’s tourism economy, which in turn is driving the kingdom’s diversification away from the reliance on oil.

But this year’s celebrations, which started yesterday, had been plagued by fears over the swine flu as well as increasing tensions with Iran, in particular its clergy and the president, Mahmoud Ahmadinejad, who announced last month that the Saudi government should not host the pilgrims because of its treatment of Shiite pilgrims and alliance with the United States.

Iran has banned all pilgrimages to Saudi Arabia during Ramadan in a bid to stop the spread of the virus.

The Haj usually plays host to 2.5 million pilgrims from across the world each year, including 200,000 from Indonesia.