The US and Turkey are working together on military plans to clear the Islamic State (IS) group from parts of northern Syria, American officials say.
They said an "Islamic State-free zone" would ensure greater stability along the Syria-Turkish border.
The talks follow a major shift in Turkey's approach to IS in recent days.
Turkey, which had been reluctant to intervene in Syria, has launched raids against IS and allowed US jets to use a Turkish base.
The Turkish operations have led to tensions with Kurdish militia forces fighting IS in northern Syria.
On Monday, Kurdish People's Protection Units (YPG) said Turkish tanks had shelled their fighters near the key town of Kobane.
Turkey said it was investigating the YPG's claim.
At the same time, Turkey has admitted launching strikes on the Kurdish PKK group in northern Iraq.
Turkey has battled PKK insurgents on its own territory in a conflict that has killed about 40,000 people since 1984.
Turkey has said it has no plans to send ground troops into Syria.
"It's an effort to defeat, destroy, degrade ISIL (IS) in northern Syria, create an area there that is ISIL-free, if I could put it that way," US State Department spokesman Mark Toner told the BBC.
Further details of the efforts by the US and Turkey to set up a buffer zone in northern Syria were revealed by unnamed officials interviewed by US media.
Under the agreement being reportedly finalised, the militants would be removed from a 109km stretch west of the Euphrates River, according to the Washington Post.
Such a deal would significantly increase the scope of the US-led air war against IS in northern Syria, the paper says.
It could also increase tensions with Kurdish fighters, such as the YPG, who control much of northern Syria and are opposed to any Turkish military intervention there, correspondents say.
On Monday, the YPG said its forces had been attacked by Turkey in the Kurdish-held village of Zormikhar, west of Kobane, on Sunday evening.
It added that one of its vehicles had also come "under heavy fire from the Turkish military east of Kobane".
If the claims are true, this will complicate matters for the coalition against IS as Western powers are co-operating with Syrian Kurds against the jihadists, says the BBC's Mark Lowen in Istanbul.
A Turkish official said its recent military operations sought "to neutralise imminent threats to Turkey's regional security" and were targeting IS in Syria and the PKK in Iraq.
"We are investigating claims that the Turkish military engaged positions held by forces other than [IS]," the official said.
Kurdish forces within Syria, he added, remain "outside the scope of the current military effort".
Meanwhile NATO is preparing to hold an emergency meeting on Tuesday to discuss the situation - at Turkey's request.
Also on Monday, Turkish police continued to arrest suspected members of IS or the PKK.
More than 1000 people have been detained in nationwide operations against the militant groups since last week, Prime Minister Ahmet Davutoglu said.
He did not say how many were suspected PKK fighters and how many are accused of belonging to IS.
The week that changed Turkey
- Monday: Thirty-two people are killed by IS-linked militants in the Kurdish-majority town of Suruc, near the border with Syria.
- Thursday: IS forces shoot dead a Turkish border guard.
- Meanwhile, the PKK reportedly kills two Turkish police officers in retaliation for Suruc and what it sees as Turkey's collaboration with IS.
- Friday: Hundreds of suspected IS supporters are arrested and properties are searched; Turkish F-16 jets, based in Diyarbakir, bomb three IS targets in Syria.
- Saturday: Turkey strikes IS and PKK targets in Syria and Iraq; the PKK says the conditions are no longer in place to observe ceasefire.
- Sunday: Car bomb attack on a military convoy in Lice in Diyarbakir kills two soldiers as strikes on targets in Iraq and Syria continue.