The sound of gunfire and what appeared to be an explosion could be heard after the convoy of around 50 Humvees and amphibious assault vehicles ground to a halt.

Some marines leapt out of their vehicles in a bid to prevent any possible ambush.

"We are still trying to secure the line and attempting to prevent an ambush," one marine told AFP.

The marine warned that if the last vehicle in the convoy was attacked then the rest were in danger of becoming sitting ducks.

US forces met little Iraqi resistance as they took over swathes of the city on Wednesday but a commander warned that irregular militia remained a threat.

SPACE.WIRE