Kurdish militia forces in Syria have accused Turkey of attacking them.
The Kurdish People's Protection Units (YPG) say Turkish tanks shelled their fighters near Kobane in northern Syria.
Turkey said it was investigating the claim but insisted that its forces were not targeting Syrian Kurds.
Turkey, which had previously not been involved in fighting in Syria, launched raids on Islamic State fighters there as well as Kurdish PKK militants in Iraq, following attacks in Turkey.
Turkey has battled PKK insurgents on its own territory in a conflict that has killed about 40,000 people since 1984. It says it has no plans to send ground troops into Syria.
The YPG said its forces had been shelled in the Kurdish-held village of Zormikhar, west of Kobane, on Sunday evening.
It added that, an hour later, one of its vehicles had come "under heavy fire from the Turkish military east of Kobane in the village of Til Findire".
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.
In a statement on Monday, the YPG said: "Instead of targeting IS terrorists' occupied positions, Turkish forces attack our defenders' positions. We urge [the] Turkish leadership to halt this aggression and to follow international guidelines."
A Turkish government official said its military operations sought "to neutralise imminent threats to Turkey's regional security" and were targeting IS in Syria and the Kurdish separatist 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".
'Turmoil and instability'
The Hurriyet Daily News quoted Prime Minister Ahmet Davutoglu as saying that Turkey's willingness to "use its force effectively can lead to consequences which can change the game in Syria, Iraq and the entire region".
The US and Turkey are reported to have agreed to work together to remove IS from the Syrian-Turkish border area.
The agreement includes a plan to drive the militants from a 68-mile (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.
An unnamed US official told AFP news agency that the aim of the reported deal was "to ensure greater security and stability" along the border.
Nato is to hold an emergency meeting on Tuesday to discuss the situation.
Turkey asked for the meeting based on Article 4 of the organisation's treaty, which allows members to request such a summit if their territorial integrity or security is threatened.
"I think it's very right and very timely to have a meeting where we address the turmoil and the instability we see in Syria, Iraq and surrounding and close to Nato borders of Turkey," Nato Secretary General Jens Stoltenberg told the BBC on Sunday.
Meanwhile the YPG took control of Sarrin, a town about 40km (23 miles) south of Kobane that had been held by IS, the Syrian Observatory for Human Rights, a UK-based group, said on Monday.
Mr Davutoglu said Turkey was prepared to work with the main Syrian Kurdish party - which has links to the PKK (Kurdistan Workers' Party) - provided it did not pose a threat to Turkey.
However, the recent raids against the PKK in northern Iraq effectively ended a two-year ceasefire.
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 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 a ceasefire
- Sunday: Car bomb attack on a military convoy in Lice in Diyarbakir province kills two soldiers as strikes on targets in Iraq and Syria continue
- PKK: Kurdistan Workers' Party - Turkish Kurdish party led by Abdullah Ocalan (jailed since 1999)
- PYD: Democratic Unity Party - PKK-aligned party in Syria
- YPG: Popular Protection Units - PYD-aligned armed force in Syria
- KRG: Kurdistan Regional Government - the official governing body of the semi-autonomous Kurdistan Region of northern Iraq
- KDP: Kurdistan Democratic Party - the dominant Iraqi Kurdish party, led by Massoud Barzani