NATO warned Russia to stay away from Turkey on Monday after the Turkish air force intercepted a Russian warplane that strayed into its airspace from Syria, underscoring the heightened risk of a wider conflagration as Russia escalates its intervention in the Syrian conflict.

The alliance of Western allies, to which Turkey belongs, also expressed concern over Russia’s continuing military buildup in Syria, urging it to halt attacks on rebel-held areas in northwestern Syria that “led to civilian casualties and did not target Daesh,” using an acronym for the Islamic State, according to a NATO statement issued after a meeting to discuss the weekend incident.

Turkish Prime Minister Ahmet Davutoglu said the Russian ambassador had informed him that the incident was a mistake, but it nonetheless contributed to the sense that Moscow’s intervention in Syria had added a dangerously unpredictable new dimension to the war.

The tensions came as U.S. officials reported intensified Russian activity in apparent preparation for some form of ground offensive against rebel-held areas of northern Syria and as they noted indications from a senior Russian official that Russia is planning to send ground forces to join the combat in Syria.

Speaking in Madrid, Defense Secretary Ashton B. Carter warned that Russia’s campaign in Syria would deepen the conflict, calling it “doomed to fail” because it was primarily aimed at propping up Syrian President Bashar al-Assad.

“It will only pour gasoline on the civil war of Syria,” he said.

Other U.S. officials called the Turkish airspace violation a deliberate provocation and the kind of unpredictable act they have worried about since Russia began its military buildup in Syria last month.

“I don’t believe this was an accident,” said a senior U.S. defense official, speaking on the condition of anonymity to discuss deliberations with Turkey and other NATO allies. “This is exactly the type of unprofessional, non-professional incident we were hoping to avoid.”

The incident in Turkey’s airspace occurred as Russian warplanes were bombing a cluster of opposition-held villages in the northern coastal province of Latakia, not far from where Turkish jets shot down a Syrian plane last year. Turkey’s Foreign Ministry said in a statement that a Russian fighter aircraft entered Turkish airspace near the border town of Yayladagi shortly after noon Saturday, then exited five minutes later after it was escorted out of the area by two Turkish F-16s.

U.S. and European leaders have complained that Russia’s six-day-old air campaign has so far overwhelmingly targeted not the Islamic State but the anti-
Assad rebellion, including groups backed by the United States and its allies.

Russia appears to be establishing what amounts to a “front line” extending from the central city of Hama to the town of Latakia, where Russian warplanes are operating out of an expanded air base, according to a senior U.S. defense official.

Here's what you need to know about Russia's airstrikes in Syria. The Russian military claims the strikes target the Islamic State, but U.S. officials say it's not helping. (Jason Aldag/The Washington Post)

U.S. officials have noted, he said, signs of an expanded Russian military buildup at an equestrian club on the outskirts of Hama. The buildup includes Russian personnel and four BM-30 Smerch multiple-rocket-
launching systems, suggesting Russia may be planning to oversee a ground offensive aimed at capturing rebel-held territory in the northwest of the country.

Rebel advances in the area earlier in the summer had spurred Russia’s decision to intervene militarily on behalf of Assad’s government, which had begun looking increasingly wobbly as the four-year-old war ground on. The rebel advances in a strategic area of northern Hama province called the Ghab plain had threatened to cut communication between the Assad family’s coastal heartland in Latakia and Damascus, the capital.

Russia has said it will not send ground forces to Syria, but it is possible that Moscow will call on irregular volunteers such as those that have fought in Ukraine to supplement the wearied and depleted Syrian army in an offensive to reclaim territory.

In Moscow, Adm. Vladimir Komoyedov, the head of the Russian parliament’s defense committee, said it was “likely” that Russian volunteers who have served in Ukraine will be dispatched to Syria to join the fight, according to the Russian Interfax news agency.

Days earlier, the head of the Russian republic of Chechnya offered to dispatch pro-Russian Chechen forces to Syria to participate in operations in that country.

The Syrian army is also helped by thousands of fighters from the Hezbollah militia of Lebanon and other militias drawn from Shiite populations in Iraq, Afghanistan and Pakistan that have been trained and funded by Assad’s other major ally, Iran.

A group of 41 rebel brigades, including Islamist battalions and many of those who have received U.S. backing, issued an appeal for help Monday from their Arab allies to counter the Russian intervention, which they called “a clear invasion of the country.”

The signatories included the Salafist Ahrar al-Sham rebel group as well as more moderate units that have received help in the form of weapons and training provided by the United States and its allies, such as Suqour al-Jabal and Tajamu al-Izza, which are among the groups targeted by the Russian airstrikes. The al-Qaeda-linked Jabhat al-Nusra was not among them, though the group also has a strong presence in the areas under Russian attack.

The United States has criticized Russia for attacking moderate rebels grouped under the umbrella of the Free Syrian Army, but it has given no indication that it is prepared to increase aid to assist them.

On Monday, Russian Foreign Minister Sergei Lavrov responded by challenging the existence of moderate rebels. He called the Free Syrian Army “a phantom group,” adding that “nothing is known about it.”

Murphy reported from Washington. Craig Whitlock in Madrid and Thomas Gibbons-Neff and Karen DeYoung in Washington contributed to this report.

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