An explosion in an open-air market in Cairo's Old City killed four people on Thursday, sources of the Egyptian Health Ministry said.

According to the sources, 18 others were wounded in the blast which occurred at around 5:00 p.m. (1500 GMT) in the market close to al-Azhar mosque, one of the most revered shrines in the Sunni Muslim world.

The French Foreign Ministry has confirmed one French woman was killed and three other French nationals wounded in the blast.

Around the mosque are a number of markets selling souvenirs and traditional handcrafts and frequented by foreign tourists.

Witnesses said a man on a motorcycle threw a bomb in the al-Moski bazaar and the bomb went off near an organized tour group in the market. While some reports said the perpetrator of the blast was a man on foot.

An Interior Ministry official told Xinhua the wounded were four French, three Americans, an Italian, a Turk and nine Egyptians.

Minister of Health and Population Mohammed Awad Afifi Tag Eddin rushed to the scene to handle the emergency, Egypt's official MENA news agency said.

Hundreds of policemen have cordoned off the area as they were trying to determine the cause of the blast.

The explosion is the second deadly attack against tourists in Egypt in six months.

Last October, Taba Hilton hotel in the Sinai peninsula and two tourist camps, 55 km further south, were hit by three bombs almost simultaneously, killing dozens of people.

Egyptian investigators found eight Egyptians and a Palestinian were behind the bombings but ruled out the possibility that they belonged to a larger terror organization.

Two of the nine suspects were killed in one of the blasts when their bomb went off prematurely, the authorities said.

Security forces arrested five of the nine suspects after the attacks, accusing them of helping the bombers carry out the deadly attacks.

The other two suspected of involvement in the bombings were killed in February in a gunbattle with police in the Sinai desert.

Egypt has largely been free from violence since the government cracked down on Islamic extremists after the killing of 58 foreign tourists and four Egyptians by gunmen in 1997 at the Pharaonic temple of Hatshepsut outside Luxor in southern Egypt.

The attack dealt a heavy blow to Egypt's tourism industry, a mainstay to the country's economy and a major hard currency earner.

Tourism has recovered in recent years. In 2004, tourism revenues hit a record high of 6.1 billion US dollars.