Ratcheting up the confrontation over the Syria war, Russia said Monday that its “volunteer” ground forces would join the fight, and NATO warned the Kremlin after at least one Russian warplane trespassed into Turkey’s airspace.
The saber-rattling on both sides reflected a dangerous new big-power entanglement in the war, as longstanding differences between Russia and the United States over President Bashar al-Assad of Syria and his opponents increasingly play out not only in the halls of the United Nations but on the battlefield in Syria.
Russia squared off with Turkey and its NATO allies, calling the air incursion on Saturday an innocent mistake because of foul weather — a claim American officials rejected.
News services said late Monday that a second airspace violation might have been committed on Sunday, but that report could not be immediately confirmed.
The Russian air and ground deployments in Syria challenge the regional policies of President Recep Tayyip Erdogan of Turkey, President Obama and NATO.
The NATO secretary general, Jens Stoltenberg, called the presence of Russian military aircraft in Turkish airspace “unacceptable” and urged Russia to align with international efforts in the region. A Russian ground force could fundamentally alter the conflict, which has left 250,000 people dead and displaced half the country’s population since it started in 2011.
Although President Vladimir V. Putin of Russia said he would not put troops in Syria, the plan for so-called volunteers was disclosed Monday by his top military liaison to the Parliament, Adm. Vladimir Komoyedov. It seemed similar to Russia’s stealth tactic in using soldiers to seize Crimea from Ukraine in March of 2014 and to aid pro-Moscow rebels in eastern Ukraine.
Moreover, American military officials said they believed that more than 600 Russian military personnel were already on the ground in Syria, not counting aircrews, and that tents for nearly 2,000 people had been seen at Russia’s air base near Latakia, in northwest Syria near the Turkish border.
Russia intensified the airstrikes it began in Syria last week, with new attacks on territory near Palmyra that is indisputably held by the Islamic State. But Russian targets remain a matter of deep contention.
Russian officials say they are targeting the Islamic State, though their bombs have mainly hit territories held by other insurgents who oppose Mr. Assad, Russia’s ally. The strikes have hit the Army of Conquest, an Islamist faction that includes the Nusra Front, Al Qaeda’s Syrian affiliate, as well as more-secular groups that often fight alongside it, including some that have received covert American aid.
The Obama administration, by contrast, says its own airstrikes against the Islamic State in Iraq and Syria can succeed only with a political transition that ends with Mr. Assad’s removal.
The administration’s position was ridiculed Monday by Sergey V. Lavrov, Russia’s foreign minister, who said the American airstrikes, which began more than a year ago, had done little militarily. In comments carried by Russia’s official Tass news agency, Mr. Lavrov said that even the Americans had acknowledged their faltering efforts to create a force of so-called moderate insurgents in Syria.
“Nobody knows about these people,” he said. “Nobody’s really heard about the moderate opposition.”
These kinds of ‘mistakes’ can easily start a war. Putin knows this. 
Foreign Minister Mohammad Javad Zarif of Iran, Mr. Assad’s regional ally, was also dismissive of the American efforts in Syria, both to unseat Mr. Assad and to combat the Islamic State. Sounding emboldened by the Russian airstrikes, Mr. Zarif said at a talk in New York that there was a difference between Russia — which was invited by Mr. Assad to help — and the American-led coalition that has been bombing Syria. “Why are you there?” he said. “Who gave you the right to be there?”
The Russian disclosure that so-called volunteer forces might soon be in Syria fueled speculation of an impending ground offensive against insurgents, one that would involve unprecedented coordination among Mr. Assad’s allies.
It could include Syria’s army fortified by forces from Russia, Iran and the Lebanese militia Hezbollah, which has deployed fighters in Syria for years to help Mr. Assad. Likely targets are Army of Conquest insurgents who threaten Mr. Assad’s coastal strongholds from territory they have seized in Idlib Province, in the north.
After the Turkish airspace incident — which, at least in theory, could have escalated into a confrontation between Russia and NATO — an Obama administration official called Russia’s behavior “deliberately provocative,” while Admiral Komoyedov said his country’s “volunteers” on the way to Syria “cannot be stopped.”
As the global powers postured, gaps deepened between local and regional participants in the war, and predictions that the Russian action would strengthen radicals in the Syrian insurgency appeared to be accurate.
