Kurdish guerrillas have moved into frontline positions abandoned by Iraqi troops in northern Iraq.
The US troops are preparing to open a northern front
The move comes shortly after hundreds of American troops parachuted into northern Iraq in what the Pentagon said was the start a northern front.
The Iraqi Government forces left positions west of the Kurdish-held town of Chamchamal, which defend approaches to the strategically important city of Kirkuk, not long after the arrival of US troops.
Kurdish guerrillas took over a ridge once manned by the Iraqi troops - but have so far not advanced further.
US troop movement
BBC correspondent Jim Muir says it is not clear whether the Iraqi troop movement was a collapse of the Iraqi line or a tactical retreat in order to better defend Kirkuk itself.
It is unclear what role Kurdish guerrillas will play in the conflict
However the main Kurdish faction controlling the area, the Patriotic Union of Kurdistan (PUK), said more than 80 Iraqi defectors had crossed to their side on Thursday.
US troops arrived early on Thursday morning at an airfield at Harir, 75 kilometres (45 miles) from the main Kurdish town of Irbil.
Reporters at the scene said the paratroopers were greeted by Kurdish guerrilla fighters, who had been expecting them, and they began digging in as soon as they arrived.
US special forces had been operating on the ground in advance of the mission, and Pentagon officials said tanks and other armoured vehicles would follow soon.
A key aim of the new northern front is to prevent the Iraqi military from being able to concentrate its defences on US-led forces in the south of the country.
However our correspondent says the first major ground action may be directed not against Iraqi troops but against the radical Ansar al-Islam faction, alleged by the US to have links to al-Qaeda, which controls some mountain territory close to the Iranian border.
It remains unclear what role the Kurdish guerrillas will play in any northern campaign.
They have put themselves at the disposal of US forces as an independent Iraqi opposition force.
But the prospects of that offer being taken up will depend on the complex negotiations between the US and Turkey.
The US had intended to funnel 62,000 troops through Turkey into northern Iraq, but the Turkish Government refused to allow an invasion from its territory.
And because of Turkish sensitivities about the Kurds moving forward, it had been agreed that they would concentrate on maintaining control of their own area.
There are also unconfirmed reports that Iraqi Government forces massacred hundreds of tribesmen at the town of Hawi Jah, near Kirkuk, after they refused to fight alongside them in the northern front.
PUK guerrillas said government forces first visited the town on Wednesday evening, then returned early on Thursday when the alleged killings took place.
However as the killings took place well inside Iraqi Government territory it is difficult to verify the report.