Sarbast Babiri, a Kurdish commander, smiled triumphantly as his men, many wearing captured Iraqi helmets, milled around him. "The Iraqi army has withdrawn to positions nine kilometres north of Mosul. They left behind heavy machine-guns, rocket launchers, food and many dead bodies," he said.

The crumbling of the northern front, quiescent since the start of the war, is a serious blow to Saddam Hussein, because he will face attacks from the north as well as the south. It may, however, increase the possibility of a Turkish invasion of northern Iraq which Colin Powell, the US Secretary of State, tried to head off yesterday.

Mr Powell, in Ankara for a one-day visit, said he was addressing Turkish concerns over "the extension of control out of Kurdish areas towards the south". As he spoke there were signs that Kurdish peshmerga (soldiers) were doing just that. But, conscious of the danger from Turkey, they portrayed their advance as unplanned, saying it was the result of a mistake by the Iraqis.

The Iraqi front line in northern Iraq was, until yesterday, 60km north of Mosul. It ran along the top of steep green hills, crowned with sandbagged bunkers, just outside the Kurdish village of Bardarash.

Commander Babiri said the Iraqi army had been relieving one of its units with another – a standard tactic apparently designed to prevent desertions and to limit the time its troops spend under air attack in their exposed hill-top positions. "The soldiers in the newly arrived unit did not know where our peshmerga front line was," he said. "They started firing at our men and we shot back."

At this point a US Special Forces detachment with the peshmerga called in air strikes on the Iraqi troops. "The Americans were with us and they were co-ordinating the plane attacks," Commander Babiri said. Over the past month US special forces have been secretly operating with peshmerga units.

Villagers in the Kurdish settlement of Kanilan, previously under Iraqi army control, confirmed that the Iraqi regiment stationed at Mandan bridge, a concrete structure spanning a small stream nearby, had suffered heavy casualties.

Hoshyar Ahmed, a villager, said: "We saw the American aircraft bomb them. Their vehicles brought away many dead and wounded. They pulled out so fast they did not even have time to blow up the bridge, although they had mined it."

Iraqi army retreats last week back to Kirkuk city were purely tactical, with no weapons left behind in the their abandoned barracks. But, as we stood outside the tiny village shop, with its assortment of cheap biscuits, in Kanilan, two peshmerga drove up in a captured Iraqi army truck and proudly opened the back flap to display a rifle they had taken.

Commander Babiri was wary of saying anything that might foster Turkish suspicions that the Kurds were deliberately moving into Mosul province. "We did not leave our positions during the fighting," he said, although it was difficult to see, if this were true, as he and his men were now six miles down the road towards Mosul.

He added: "We will only go as far as there are Kurdish lands," referring to the Kurdish territories from which they were driven over the last 30 years by the Iraqi government's systematic ethnic cleansing.

The peshmerga suffered only five dead while villagers said that as many as 200 Iraqi soldiers were killed or wounded by the air strikes. This underlines the fact that the Iraqi army in open country cannot withstand even lightly armed Kurdish infantry supported by US air power.

In reality, the peshmerga do not have to advance into Kirkuk or Mosul. Once the Iraqi army retreats or breaks up, about 300,000 Kurdish refugees from the two provinces – many of them armed – have said they intend to return home as soon as possible. And it is becoming increasingly difficult for Turkey to invade because of the growing number of US troops in northern Iraq.

US aircraft repeatedly raided the Iraqi front line in Mosul and Kirkuk provinces over the past week. High over Mosul yesterday were the vapour trails of B-52 bombers, and plumes of smoke rose from the general direction of the city.

On the ridge opposite the town of Kalakh, south-east of Mosul – probably the most-filmed military position in Iraq because many television companies have rented houses in the town – Iraqi soldiers were still visible yesterday, moving about around their frequently bombed bunkers.

Yet the soldiers appear to be under orders not to do anything to ignite the northern front by opening fire, even under provocation. One of the main roads linking the Kurdish capital Arbil with the western city of Dohuk goes through Kalakh across a bridge over the Zaab river and then runs along one of the banks of the river, overlooked by an Iraqi machine-gun post on a hill 100 yards away.

Although the Kurdish leaders say they are now part of the US-led coalition, the Iraqi gunners have never opened fire on the road. Yesterday, cars were passing there freely.

But with the Iraqi army retreat from Bardarash back to Kirkuk, the forces in Kalakh are vulnerable to being cut off from the rear.

It is difficult to see how they can stay in their present positions. There is also an increased flow of deserters, trying to avoid the relentless bombardment, to the Kurds.