By Maya Gebeily
NATO has called on Russia to stop air strikes in Syria and warned its violation of Turkish airspace during a raid risked inflaming tensions.
Turkey warned Moscow 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 (local time), conducting air strikes on what Moscow says are targets belonging to Islamic State jihadists 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 on Monday 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.
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 IS 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 IS targets on Monday, adding that 10 facilities had been hit.
The United Nations warned on 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 IS-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 on 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.
Some of the statement's signatories, such as the Islamist Ahrar al-Sham and moderate Jaish al-Izzah, have been targeted by Russia.
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.