Iraq has called on Turkey to immediately withdraw its troops from Iraq's north, calling their presence a violation of Iraqi sovereignty.
Prime Minister Haider al-Abadi said no permission had been given for the stationing of "around one armed battalion" of Turkish soldiers in the northern Nineveh area near Mosul.
Mosul is under the control of the Islamic State of Iraq and the Levant (ISIL) group.
Al Jazeera's Imran Khan, reporting from Irbil, said it appeared the Turkish troops had crossed into Iraq at the invitation of the governor of Mosul, Atheel Nujaifi.
"I can confirm that a number of Turkish troops have crossed the border into northern Iraq and it looks like these forces were invited by Nujaifi, who runs a militia called Hashd al-Watani (National Mobilisation Front) on the outskirts of Mosul," Khan said.
"This latest development has caused a major row between Baghdad and Ankara. And this is just the latest one in a long list of disagreements. They have been arguing over oil revenues from the Kurdish region for a while now.
"It seems like the government of Mosul has told Baghdad: 'We need more help with fighting ISIL, and if the Turks are willing to offer that help, we will take them up on that'."
Iraq's foreign ministry in a statement on state television described the Turkish deployment as "an incursion" and rejected any military operation that was not coordinated with the government in Baghdad.
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A Turkish security official said the troops had been in the Kurdish region of northern Iraq on a training mission since last year.
An anonymous Turkish security official told Reuters news agency the troops had been in Iraqi Kurdistan and had moved to Mosul accompanied by armoured vehicles - a move that coalition countries targeting ISIL were aware of.
Video released on the website of Turkey's pro-government Yeni Safak newspaper showed flatbed trucks carrying armoured vehicles along a road at night, describing them as a convoy accompanying the Turkish soldiers to Bashiqa.
US aware
A senior Kurdish military officer based on the Bashiqa front-line, north of Mosul, said additional Turkish trainers had arrived at a camp in the area - run by Hashd al-Watani - overnight on Thursday escorted by a Turkish protection force.
He said he was not aware of the size of the force and refused to speculate.
"Our soldiers are already in Iraq. A battalion of soldiers has gone there. Training was already being given in that region for the last two to three years. This is a part of that training," a senior Turkish official said.
In Washington, two US defence officials said on Friday that the United States was aware of Turkey's deployment, but the move was not part of the US-led coalition's activities.
Turkey has close relations with the Kurdish autonomous region of northern Iraq, though it views Syrian Kurdish groups as hostile to its interests.
"This is part of the fight against Daesh [ISIL]," the Turkish official said, adding there were about 20 armoured vehicles accompanying the force as protection.
ISIL occupies swaths of Iraq and Syria, profiting from disunity among groups opposing it.
On Tuesday, the US said it was deploying a new force of special operations troops to Iraq to conduct raids against ISIL there and in neighbouring Syria, ratcheting up its campaign against the group.
 
Source: Al Jazeera And Reuters