They were initially engaged by small arms fire by a US marine patrol. They continued to return fire and the captain, Captain Rick Crevier, called in the tanks.
Prisoners of war
Then they received a radio report that four prisoners of war had been captured.
I'm not sure whether this was involved in this incident or somewhere else.
This is in a largely deserted, semi-industrial area.
This continued for more than an hour before four M-1 Abrams American battle tanks were called in.
They fired into a compound in which the Iraqi Republican Guard were thought to be concealed.
Return fire continued to come from inside the complex and the marine company commander Captain Rick Crevier summoned air support.
Two 500 pound bombs were dropped, one hitting the target.
The first bomb was wide of the target. I had to seek shelter and did not see the second, but commanders tell me it did hit the position.
No American was killed or injured. It is not yet known how many Iraqi dead there are.
US marines are moving in to see the result of the bombing.
The presence of so many Republican Guard has come as a great surprise to US marines.
Efforts are continuing in Umm Qasr to clear the town of pockets of resistance so the port can be opened to let humanitarian aid into the country.
Saturday was a frustrating day for US marines in Umm Qasr.
The main new port facility was secured by marines after they finished scouring more than 50 buildings in the port complex, checking offices and behind containers and through railway carriages in the sidings.
The same was broadly true of the old port, one-and-a-half miles to the south of the new port.
Extra measures will have to be taken before that can happen, including checking that the waters have not been mined.
Also, there are suggestions that a number of ships may have been scuttled in some of the deeper channels.
A team of divers is expected to move in over the next few days.