By Jonathan Marcus BBC defence correspondent at Central Command HQ in Qatar

While Iraqi soldiers have put up little resistance in the south, irregulars are fighting a guerrilla war

While regular Iraqi units in the south seem to have offered only sporadic resistance, many just melting away ahead of the US and British advance, groups of Iraqi irregulars have emerged from hiding to ambush coalition forces.

These fighters are thought by sources at the Central Command headquarters to be drawn from organisations like the Fedayeen Saddam, or the special security organisation.

US commanders may be missing the heavier mechanised division that is still at sea, which should have entered Iraq from Turkish soil

Both groups are intensely loyal to the Iraqi regime.

They are lightly armed and many are said to be fighting in civilian clothes, taking sanctuary in populated areas like Nasiriya, Basra and Umm Qasr.

These fighters were clearly moved into southern Iraq ahead of the start of the US and British campaign.

They are fighting a classic guerrilla or irregular war.

The problem for US and British commanders is that the pace of their own advance has been remarkable, but their lines of communication are insecure.

Unarmed, so-called soft skin vehicles have to move up behind the lead combat formations to carry out resupply and repairs.

They are vulnerable.

Risky strategy

Nonetheless, further to the north-west, the first clashes are occurring between the US Army and Republican Guard formations.

These Iraqi units are already coming under attack from US and British war planes and helicopters.

The southern battle is, however, only one part of the mosaic of operations that may be under way.

We know little about what is going on in the north and west of Iraq.

But the US and British strategy was a risky one from the outset.

They have a relatively small combat force, a significant proportion of which are lighter troops like the US Marines and airborne units.

US commanders may be missing the heavier mechanised division that is still at sea, which should have entered Iraq from Turkish soil.