After reporting dramatic breakthroughs Wednesday against Iraq's elite Republican Guard south of Baghdad, US troops appeared poised for the endgame of their campaign to topple President Saddam Hussein.

But stung by the surprising opposition and casualties in their drive north through the Iraqi desert, and with fears persistent that Saddam could resort to chemical weapons if cornered, the Americans were moving warily.

"We are not expecting to drive into Baghdad suddenly and seize it," Army Major General Stanley McChrystal, vice director of operations of the Joint Staff, said Wednesday.

US Defense Secretary Donald Rumsfeld stressed the battle was far from over and the American troops faced "some difficult days ahead and dangerous days ahead in terms of fighting."

Amid suggestions they might have misjudged the Iraqis, US commanders were planning to more than double their ground combat strength to 210,000 soldiers. But they also point to a string of successes since the conflict started March

They have made rapid progress through the desert and control more than a third of Iraq, including oil fields in the south. They have avoided so far a Turkish incursion in the north and Iraqi attacks on Israel that could widen the war.

"We are in fact on plan and where we stand today is not only acceptable in my view, it is remarkable," the top US commander for the Middle East, General Tommy Franks, said last weekend in response to criticism of his tactics.

But the lightning invasion has exacted a cost.

The number of civilians casualties is unknown, although the Iraqi government has put it at over 500.

Among the confirmed deaths are seven women and children shot at a US checkpoint Monday and 33 killed Tuesday in what looked like an allied cluster bomb strike on a town south of Baghdad.

Hundreds of Iraqi fighters -- uniformed troops, paramilitaries, civilian militiamen and members of the ruling Baath party -- have been killed by coalition strikes, according to tallied accounts from US and British officers.

Of the nearly 100,000 US and British troops in Iraq, more than 75 have been killed, according to the most recent tolls given by Washington and London. Most of the 27 British military deaths have been through helicopter accidents or "friendly fire" incidents.

But the US commanders have also met with some surprises in the field.

Iraqi forces paraded five US captured soldiers and the bodies of at least four others, all members of a lost supply convoy bringing up the rear.

At least three other US prisoners of war were taken in that and later engagements, although one, a 19-year-old female private, Jessica Lynch, was rescued overnight Tuesday.

A suicide attack at a US checkpoint in central Iraq that killed four GIs on Saturday, Baghdad's warning that 3,000 Arabs had arrived in Iraq prepared for similar "martyrdom", and accounts of Iraqi fighters feigning surrender to attack coalition troops have all made coalition soldiers skittish.

At the same time, the welcome from civilians has been ambivalent, with some apparently fearing that Saddam's regime would survive, some wanting it to, and others declaring that the Anglo-American invaders were simply not wanted.

There have certainly been some scenes of friendly greeting of the troops, but they have fallen short of the scenes of rapturous cheering of the "liberators" US officials had predicted and hoped for in the run-up to the war.

Tactical changes have occurred as a result, including more cautious checkpoint procedures and POW vetting.

The senior field commander in Iraq, US Lieutenant General William Wallace, admitted last week that the enemy was "different from the one we'd war-gamed against".

Coalition forces have skirted around population centres to avoid the nasty street-by-street combat which Saddam has been counting on to slow their progress.

Even as they stood at the door to Baghdad, US and British forces had not as yet taken a single major town or city along the way.

British soldiers -- arguably the most experienced in urban combat because of Northern Ireland -- have besieged Iraq's second city of Basra in the south for over a week but have been unable to dislodge Saddam loyalists.

The small port of Umm Qasr is fully under their control, though, allowing them to bring in the first shipment of humanitarian aid.

Those tonnes of food, water and medicine, along with air drops of millions of leaflets urging surrender and propaganda radio transmissions, are designed to sap the Iraqis' will to fight and reassure populations that they would be looked after.

WAR.WIRE