As the US 3rd infantry's 2nd brigade was mounting its high-profile incursion into the heart of the capital, at least two of its soldiers and two journalists were killed when the brigade's tactical operations centre on the city's southern outskirts was hit by a single rocket.

One of the dead journalists was identified last night as Julio Anguita Parrado, 31, a New York correspondent for the Spanish newspaper El Mundo. The identity of the other journalist had not been revealed but the German news channel n-tv was reporting last night that he was German.

The death toll may rise further. A further 15 people were wounded, two of whom remained in a critical condition last night. CNN said it had been told by Pentagon sources that six soldiers initially listed as missing after the attack may have been killed.

US commanders said the rocket appeared to have been fired not from inside Baghdad but from the south, indicating that Iraqi units are still operating behind US lines.

Lt Col Peter Bayer said 17 military vehicles had been destroyed by the rocket which had left a "sizeable crater".

He added: "We had a team of doctors and medics right there, which was the good news. They saved a lot of lives."

Military analysts warned that unless the Iraqis had scored a lucky hit, the attack challenged coalition claims that the forces defending Baghdad had been reduced to a rabble that was incapable of mounting coherent and effective resistance.

It provided a graphic lesson for coalition commanders that Iraqi forces loyal to Saddam still had the capability to detect a key US command post, to communicate that information to a unit and to launch a missile with enough accuracy to cause so much devastation.

According to CNN correspondent Walter Rodgers, embedded with the 3rd Infantry's 7th Cavalry regiment, the communications centre consisted of drawn-up Bradley fighting vehicles and Abrams tanks and was the key command post for the main unit involved in fighting in Baghdad.

The vulnerability of coalition forces to guerrilla attacks was made clear during the opening phase of the war, when Iraqi fighters inflicted heavy losses and slowed the push north towards Baghdad by repeatedly attacking the flanks and rear of US and British columns.

Mr Parrado, the second El Mundo correspondent to have been killed in a war zone in little over a year, had been covering the 3rd Infantry's advance on Baghdad since the start of the war. Because he was based in the US, like many of the foreign correspondents embedded with the US army, he was able to do their training courses for journalists.

He had used his second surname as a byline so as not to be confused with his father, Julio Anguita, who was the former leader of Spain's communist-led United Left coalition.