A bomb has exploded in a Cairo tourist bazaar, killing four people, among them a US citizen and a French national, and wounding 18 others, Greater Cairo's security director said.

An earlier Egyptian Interior Ministry statement said the wounded - four French, three Americans, an Italian, a Turk and nine Egyptians - were taken to hospital where the French woman died.

A US official said in Washington at least three Americans were injured in the attack.

Police sources said a man on a motorbike threw the bomb into the busy tourist area in the centre of the Egyptian capital but the subsequent ministry statement made no mention of what caused the blast.

One witness said it appeared a man on a motorcycle set off a bomb in the middle of a group of foreigners.

Rabab Rifaat, an Egyptian woman who was shopping in a store several metres from the blast, said the explosion appeared to have been caused by a man who was either carrying a bomb or had it on his motorcycle.

She said she heard "a boom, a horrible sound, very loud. Everyone started running", and that she then saw a decapitated head flying through the air. 

She said she saw six or seven bodies on the ground, some of them foreign-looking, but it was unclear if they were dead or wounded.

The last major anti-government attacks came in late 1997. In September of that year, two armed men fired automatic rifles at a tour bus parked outside the Egyptian Museum in central Cairo, killing 10 people - mostly German tourists. 

A month later, assailants killed 58 foreign tourists and four Egyptians in an attack in Luxor, southern Egypt, the worst such attack ever. 

And last October, violence against foreigners returned after explosions hit several hotels in the Sinai peninsula, including one in the resort of Taba, popular with neighboring Israelis on short visits.

The blast killed 34 people and Egyptian authorities linked the attack to Israeli-Palestinian conflict.

PHOTO CAPTION

Ambulances and rescue teams work at the site of an explosion in Cairo April 7, 2005. (REUTERS)