Final countdown for Baghdad

American forces are on the brink of the decisive push to Baghdad as the 3rd Infantry Division engaged in its first full-scale ground battle with the Republican Guard's Medina Division south-west of the Iraqi capital.

After more than a week of massive aerial bombardments on heavily dug-in Republican Guard positions ringing Baghdad, the ground advance by US forces from their front line around 50 miles south of the city was said by sources at US Central Command in Qatar to be under way. US ground troops engaged in intense battles with the Republican Guard near the holy Shia city of Karbala, which is strategically critical to the assault on the capital. If they win this battle, the way to Baghdad is open.

"This is the big battle," a US military official said. Asked if the fighting represented a new push towards the Iraqi capital, the official said: "It well could be."

Ground skirmishes with the Medina Division over the previous 24 hours launched by American "probing patrols" were described by US commanders in Iraq as the first engagements in the battle for Baghdad.

Overnight a battle raged around Karbala, as American forces sought to encirlce the town. East of Baghdad, a force of US marines was also advancing rapidly towards the capital, according to a BBC journalist who was travelling with them. The US is expected to surround the capital.

Troops from the Nebuchadnezzar Division of the Republican Guard have already moved south from Saddam Hussein's home town of Tikrit. High-ranking US officers said this was a reinforcing move because of the "rapid depletion" of the Medina Division.

Some of the division's troops were captured on Monday in fierce fighting across the Euphrates at Hindiyah. General Richard Myers, chairman of the Joint Chiefs of Staff, said in Washington that "serious combat" was already under way. There would be "bigger pushes that will be underway as soon as we're ready" he said.

Donald Rumsfeld, the US Defence Secretary added: "The circle is closing."

The Medina Division, one of the best trained and equipped in the Iraqi army, was claimed by Allied sources to be "50 per cent degraded" by daily bombing and missile attacks, which reached their climax at the beginning of this week.

Last night warplanes also bombed targets in the capital including one of President Saddam's palaces and Iraq's Olympic headquarters, also thought to house a torture centre run by one of his sons.

The decision to advance on Baghdad came as a call for an Iraqi "jihad" ­ presumed to be from President Saddam ­ was read out on Iraqi state television by Mohammed Saeed al-Sahaf, his Information Minister. The statement declared that "those who are martyred will be rewarded in heaven". It also described the Allied forces as "aggressors, evil, accursed by God".

The ground advance on Baghdad is expected to be covered by air support from RAF and US Marine Harriers, and US Apache and US Marine Cobra attack helicopters.

The push forward will probably include units of US Marines that have advanced to the east of the 3rd Infantry. Its pace may partly be dictated by the level of resistance encountered by troops in the gap of about 20 miles between the Allied front line and the Republican Guard positions. Specialist biological and chemical detection and decontamination units from the British Army are known to be with US troops in the advance.

Reuters reporters travelling with troops close to the head of the military advance reported that US forces had resumed their northwards push after a pause of several days.

One correspondent, Sean Maguire, said: "It seems as though the operational pause in our sector is over. We've swung from passivity to activity quite quickly." He said advancing troops had reported "some contact" with Iraqi forces but not "major opposition so far". His colleague Luke Baker, travelling with other divisions, said: "We are definitely on the move."

Earlier, the day had been dominated by the backlash after Monday's fatal shooting of a group of women and children at a US Army checkpoint near Najaf. And at least 11 more people, nine of them children, were killed in the central town of Hillah yesterday.

Iraqis said they were the victims of American bombing. Shocking television images from the scene, of babies cut in half and children with amputation wounds, were filmed by Reuters and Associated Press after the Iraqis allowed them to take cameras into the town.

Fears were growing last night for up to 10 British soldiers who the Iraqis claimed had been killed in northern Iraq near Mosul. Mr Sahaf said Iraqi forces had thwarted a British landing in the Baaj district west of the city.

Pictures of Iraqis apparently driving a British Land Rover were shown on the al-Jazeera television network. Mr Sahaf said the Fedayeen militia "captured most of their equipment, their weapons, their armoured cars and vehicles".