The annual Muslim Hajj pilgrimage has begun in Saudi Arabia, with officials saying they expect lower numbers mainly because of fears about swine flu.

A senior Saudi health ministry official told the BBC that international visitor numbers might not be affected, but a 40% drop in local pilgrims was likely.

The BBC's Shahzeb Jillani says it is the first official recognition that fear of swine flu may affect the Hajj.

About two million Muslims converge on Mecca each year for the pilgrimage.

Crowds of pilgrims have been converging on the holy city of Mecca in recent days, and started the Hajj rites on Wednesday.

They begin with Tawaf, walking seven times round the Kaaba - the cube-like building in the centre of the Great Mosque, Islam's holiest shrine - in an anti-clockwise direction.

Pilgrims also run seven times along a passageway in the Great Mosque, commemorating a search for water by Hajar, wife of the Prophet Abraham.

Pilgrims then travel to the valley of Arafat and stand in the open praising Allah and meditating on Thursday.

Saudi health authorities announced at the weekend that four people attending the Hajj had died from the H1N1 virus, but played down the risk to other pilgrims.

"There is no risk of the illness spreading as we are well-prepared and have taken the necessary measures," health ministry spokesman Dr Khaled Marghlani told a news conference on Tuesday.

The ministry said the four who died were a Sudanese man, a Moroccan woman and an Indian man - all aged over 75 - and a girl aged 17 from Nigeria.

The statement said the four had not followed "recommended procedures, especially vaccination against swine flu".

The ministry said 16 other cases had been detected and four were in a critical condition.

The hajj has been the scene of several tragic accidents caused by huge crowds.

In 2006, 364 people were killed in a stampede at the entrance to the Jamarat Bridge in Mina.

In a bid to avoid a repeat of the disasters, authorities have just completed the rebuilding of the bridge.

Officials say the 950m (3,135ft) long, 80m (260ft) wide five-storey pedestrian walkway, which cost $1.2bn (£723m), will prevent overcrowding.