Nato is holding an emergency meeting in Brussels to discuss Turkey's military campaign against the Islamic State (IS) group and Kurdish militants.
Turkey was until recently a reluctant partner in the US coalition against IS, but over the past few days it has bombed IS and Kurdish positions.
Turkey is also backing plans for a buffer zone on the border with Syria.
Turkey's prime minister said he expects his country's allies to show solidarity and support for its campaign.
Meanwhile Turkish police have continued to arrest suspected members of IS, the PKK and leftist groups - more than 1,000 over the past week.
The US and Turkey are working together on plans to create an "IS-free zone" along the border with Syria.
As well as targeting IS militants, the zone would also allow Turkey to hit positions held by the outlawed Kurdish PKK group.
Turkey says it draws no distinction between the PKK and IS, considering them both terrorist organisations.
- 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
Turkish President Recep Tayyip Erdogan said the creation of a "safe zone" would pave the way for the return of Syrian refugees.
Turkey is struggling with more than 1.8 million refugees from the Syrian conflict.
Under the plan, militants would be removed from a 68-mile (109km) stretch west of the Euphrates River, officials say.
Such a deal would significantly increase the scope of the US-led air war against IS in northern Syria, say analysts.
The crisis has exacerbated tensions in a part of Turkey where a conflict between the outlawed PKK and government troops has killed about 40,000 people since 1984.
Last week Turkey agreed to allow the US to use its air base at Incirlik to launch air strikes against IS.
Turkey called the meeting of ambassadors to Nato under a clause allowing members to request a summit if their territorial integrity or security is threatened.
It is only the fifth such request in the alliance's history.
Nato Secretary General Jens Stoltenberg said as he arrived for the talks: "Terrorism in all its forms can never be tolerated or justified.
"It is right and timely that we hold this meeting today to address the instability on Turkey's doorstep and on Nato's border."
'Outside the scope'
Turkey's cross-border strikes followed a wave of attacks by militants on Turkish soil.
In the worst incident, IS-linked militants killed 32 people in the Kurdish-majority town of Suruc, near the Syrian border on 20 July.
Analysis: Mark Lowen, BBC News, Istanbul
The Turkish government has long been accused of at best turning a blind eye to the rise of IS - and at worst, actively backing the jihadists against the Assad regime. It has always denied the allegation.
But last week came the suicide bombing in Suruc, southern Turkey, in which 32 died and which Turkey blamed on a militant trained by IS.
Ankara's strategy is complex. Alongside the IS strikes, Turkey has now bombed several PKK positions and arrested hundreds of suspected members of the group.
Critics believe Turkey is only striking the jihadists as cover for going after its real enemy: Kurdish militants.
Ankara's reluctance to hit IS earlier, the argument goes, was actually a reluctance to help Kurds fighting IS militants. Now both can be bombed, Turkey is willing to get involved.
Turkey insists Syrian Kurds are "outside the scope of the current military effort".
But overnight on Monday, the YPG Kurdish armed force in Syria said Turkish tanks hit its positions in a village in Syria's Aleppo province, wounding fighters and villagers, reported the AFP news agency.
Meanwhile, the PKK was suspected of shooting dead a military police commander in the south-eastern Turkish province of Mus late on Monday.
As Turkey and the PKK continue tit-for-tat attacks, the possibility grows of a return to armed conflict, our correspondent warns.
If that happens, Turkey will be plunged into a serious crisis and the West's strategy against Islamic State could be severely hampered.
The week that changed Turkey
- Monday 20 July: 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