BEIRUT (AFP) - NATO on Monday (Oct 5) called on Russia to stop air strikes in Syria and warned its violation of Turkish airspace during a raid risked inflaming tensions days after Moscow's military intervention began.

Turkey also warned Moscow over further incursions after its F-16 jets intercepted a Russian fighter that flew through its airspace near the Syrian border at the weekend.

Two Turkish jets were also harassed by an unidentified MIG-29 on the Syrian border according to Turkey's army, which has the second-largest number of troops in NATO after the US.

"Our rules of engagement are clear whoever violates our airspace," Turkish Prime Minister Ahmet Davutoglu said.

"The Turkish Armed Forces are clearly instructed. Even if it is a flying bird, it will be intercepted," he added, while playing down the idea of "a Turkey-Russia crisis".

Russian warplanes have been flying over Syrian territory since Wednesday, conducting air strikes on what Moscow says are targets belonging to Islamic State in Iraq and Syria (ISIS) militants in the country's northern and central provinces.

The West has accused Moscow of using the raids as cover to hit moderate opponents of Russian ally Syrian President Bashar al-Assad.

After holding an emergency meeting of its 28 member states, Western military alliance NATO called on Russia to "immediately cease its attacks on the Syrian opposition and civilians".

It also warned against violating Turkey's airspace, saying in a statement after the meeting that the allies "note the extreme danger of such irresponsible behaviour." US Secretary of State John Kerry said the incident had risked provoking a serious escalation.

"We're greatly concerned about it because it is precisely the kind of thing that had Turkey responded under its rights could have resulted in a shoot-down," he said.

- 'Fraught with danger' -

Russia later played down the incursion, saying one of its aircraft had briefly entered Turkish airspace as a result of "unfavourable weather conditions".

"There is no need to look for some conspiracy theories," a defence ministry statement said.

Turkey and Russia remain on opposing sides of the Syrian conflict, with Moscow one of the few allies of Assad while Ankara backs a solution excluding the embattled leader.

Turkey has stepped up its role in a US-led coalition that has been targeting ISIS for a year as violence in Syria and Iraq has increasingly been spilling over its borders.

Russia said its warplanes had carried out 15 sorties on 10 ISIS targets on Monday, which destroyed an ISIS command centre and communications hub in a mountainous area of Damascus province, the statement said.

They also destroyed ISIS command centres in the northern province of Aleppo, around 20 tanks and two ammunition depots in the eastern province of Homs, as well as 30 vehicles, including armoured vehicles, in northwestern Idlib province.

The United Nations warned Monday of the risks of having so many powers operating in Syrian airspace.

"It creates a situation that is fraught with danger and very delicate as we had seen on the issue of the violation of airspace with Turkey," said spokesman Stephane Dujarric.

Turkey has pushed for a so-called ISIS-free zone in northern Syria that could provide safe haven to refugees, but Russia has opposed the move, saying "it is necessary to respect countries' sovereignty".

More than 40 of Syria's most powerful rebel factions said Monday Russia's air campaign had "cut the road to any political solution" and urged a regional coalition to fight the regime and its allies Russia and Iran.

- 'Losing strategy' -

US Defense Secretary Ashton Carter said Russia was pursuing a "losing strategy" in Syria.

"Russia has escalated the civil war, putting further at risk the very political resolution and preservation of Syria's structure of future governance it says that it wants," he said.

ISIS has seized large parts of Syria and neighbouring Iraq, committing atrocities including beheadings, rape and mass killings, and destroying archaeological and cultural heritage.

On Sunday the militants blew up the Arch of Triumph in Syria's ancient city of Palmyra, the country's antiquities director Maamun Abdulkarim said. "This is a systematic destruction of the city. They want to raze it completely," Abdulkarim told AFP.

UNESCO chief Irina Bokova denounced the latest demolition, which comes after the group razed other parts of the site which it seized in May, including the 2,000-year-old Temple of Bel.

"Extremists are terrified by history and culture," she said, vowing there would be "no impunity" for those responsible.

Under ISIS's extreme interpretation of Islamic law, the ancient shrines are considered idolatrous and must be destroyed, although the group has also smuggled and sold antiquities.

Analyst Charlie Winter of the London-based Quilliam Foundation think tank said such destruction was a "low-risk, cheap" way for ISIS to raise its profile among potential new recruits and grab headlines.