NATO's chief accused Russia on Tuesday of deliberately violating Turkey's airspace during bombing raids on Syria, as Ankara summoned Moscow's envoy over the latest intrusion.
The rise in tensions came as Russian warplanes struck Islamic State group targets in and around the ancient city of Palmyra for the first time, Syrian state television said.
NATO member Turkey accuses Russia of violating its airspace at least twice over the weekend, and has warned it will activate its rules of military engagement if such intrusions are repeated.
NATO has criticised the violations as "irresponsible" and urged Moscow to end raids against civilians and Syria's opposition forces.
On Tuesday, it stepped up the rhetoric, insisting the violations were intentional.
"For us, this does not look like an accident, it is a serious violation," NATO chief Jens Stoltenberg said at a press conference in Brussels.
"The violations were for a long time compared to previous violations of airspace we have seen elsewhere in Europe," he said.
Turkey meanwhile said it had summoned the Russian ambassador to Ankara for the second time in as many days to protest a violation of its airspace on Sunday.
Turkey warned that "Russia would be held responsible" for future intrusions, a Turkish official said.
- Violations 'irresponsible': NATO -
Ankara said that on Saturday its fighter jets intercepted a Russian warplane and forced it to turn back.
Then on Sunday, two Turkish F-16 jets were "harassed" by a MIG-29 plane, according to the army.
"Our rules of engagement are clear whoever violates our airspace," Turkish Prime Minister Ahmet Davutoglu warned on Monday.
"The Turkish Armed Forces are clearly instructed. Even if it is a flying bird."
Russia has acknowledged the Saturday incident, which it blamed on "unfavourable weather conditions".
"There is no need to look for some conspiracy theories," the defence ministry said of the incident, which it added lasted "several seconds".
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- 'First stage' for Russia -
Russian warplanes have been carrying out raids in Syria since Wednesday, targeting what they call "terrorist" groups, though rebels and their backers accuse Moscow of hitting a range of opposition fighters, not just jihadists.
On Monday, NATO condemned the incursions into Turkish airspace and noted the "extreme danger of such irresponsible behaviour".
The alliance also urged Russia to "immediately cease its attacks on the Syrian opposition and civilians" and focus instead on hitting IS.
Late Monday, Russia's defence ministry said its jets had struck 10 IS targets in Damascus, Aleppo, Homs and Idlib provinces throughout the day.
On Tuesday, a military source cited by Syrian state television said Russian forces struck the city of Palmyra and its outskirts, "destroying 20 armoured vehicles, three ammunition warehouses, and three rocket launchers."
Lebanese newspaper Al-Akhbar, citing a senior Syrian officer, said the Russian strikes were "in this first stage targeting concentrations of armed groups at the intersection points of different provinces".
The Russian operations "are still in the first stage, but will expand as part of an operation lasting several months," a Syrian military source told AFP.
The source said it was too early to talk about new ground operations, "because now is the time for air operations carried out by the Russians".
The Russian operations come more than a year after a US-led coalition began bombing IS in Syria, without coordinating with Damascus.
The United Nations warned Monday that having so many powers operating in Syria "creates a situation that is fraught with danger and very delicate".
And in a sign of the complex interests involved, Israel's military confirmed it would be holding talks with a visiting Russian delegation on a mechanism to avoid accidental clashes in Syria.
IS has taken advantage of the chaos that has engulfed Syria since an uprising began in March 2011 with protests against Assad's regime.
It holds large swathes of territory in Syria and neighbouring Iraq, where it commits atrocities including beheadings, rape and mass killings.
The group has also destroyed archaeological and cultural heritage that it considers idolatrous in acts calculated to garner headlines and propaganda value, experts say.
On Sunday, the jihadists blew up the Arch of Triumph in Palmyra, prompting new outrage at the destruction of the UNESCO World Heritage site.
"This is a systematic destruction of the city. They want to raze it completely," Syria's antiquities chief Maamun Abdulkarim told AFP.