Reporters counted four corpses outside the bus and Marines said another 16 lay inside. All the bodies were adult men, wearing a mixture of civilian and military clothing and had papers that appeared to identify them as Iraqi Republican Guard.

The Marines were searching the luggage for weapons, but one told Reuters that all that was found were two small handguns.

Two men who survived the attack, which appeared to have taken place on Wednesday, were being tended by Marine medics and being prepared for evacuation to hospital. Other survivors had apparently escaped into nearby fields and palm tree plantations.

One Marine picked up a wallet. Inside were wedding pictures, an army membership card and a picture of the England World Cup soccer team.

Cannon and machine-gun fire had peppered the bus with holes, killing most of the passengers in their seats. The grisly remains were evidence of the ruthlessness with which lead Marine units are clearing the road north of the central city of Nassiriyah to make way for a huge military convoy.

Reporters have seen more than a dozen burned-out trucks and buses and the corpses of at least 60 Iraqi men lying beside them during a three-day push out of Nassariyah, where Marines suffered up to 10 fatalities during ambushes on Saturday.

The US deaths, the largest daily toll so far in the weekold war, have put the Marines on the defensive. Since the weekend they have laid down heavy covering fire as they go through towns, while insisting they are taking care to spare civilians.

Pick-up trucks with Iraqi families have been seen on the road, untouched by the fast-moving, scouting vehicles that have picked off the trucks and buses with their heavy cannon and machine-guns. But civilians have died in crossfire in towns.

It was impossible for reporters to see if any of the drivers or passengers killed in the road attacks were civilians. US officers said they believed the dead were men being sent to reinforce the defense of Iraqi towns.

US officers inside this small town 220 km southeast of Baghdad said the threat they faced from irregular forces trying to stage ambushes required an aggressive approach. Brig. Gen. John Kelly, visiting the front-line forces, said he had himself seen armed men dismounting from civilian buses.

“That’s the problem we face here. We have very little time to decide if a truck or bus is going to be hostile,” he said.

Resistance by Iraqi irregular units have slowed the Marine advance and forced them into skirmishes in urban areas that they would have preferred to avoid.

Marine forces are heading north toward Kut, a major city on the Tigris river some 170 km southeast of Baghdad, in an apparent bid to pressure the capital from its eastern flanks. Kut was the scene of a major British defeat in World War One.

Kelly said Iraqi civilians have generally been “very positive” toward the invasion force. But the Marine defensive tactics used in Qalat Sukkar and other towns, where houses fronting main streets were raked with fire and tall buildings singled out for a pounding as potential observation posts, will have won them few friends.

As the convoy left Qalat Sukkar yesterday a large crowd of Iraqi men and children gathered on the main highway behind them. The crowd said nothing, just silently watched as their violent overnight guests departed.