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.

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MOSCOW — 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 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 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 U.S. 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.

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 Putin of Russia said he would not put troops in Syria, the plan for so-called volunteers was disclosed on 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, U.S. military officials said they believed 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 group. But Russian targets remain a matter of deep contention.

Russian officials say they are targeting the Islamic State group, though their bombs have mainly hit territories held by other insurgents who oppose Assad, Russia’s ally. The strikes have hit the Army of Conquest, an Islamist faction that includes the Nusra Front, al-Qaida’s Syrian affiliate, as well as more-secular groups that often fight alongside it, including some that have received covert U.S. aid.

The Obama administration, by contrast, says its own airstrikes against the Islamic State group in Iraq and Syria can succeed only with a political transition that ends with Assad’s removal.

The administration’s position was ridiculed on Monday by Sergey Lavrov, Russia’s foreign minister, who said the U.S. airstrikes, which began more than a year ago, had done little militarily. In comments carried by Russia’s official Tass news agency, 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.”

Foreign Minister Mohammad Javad Zarif of Iran, Assad’s regional ally, also was dismissive of the U.S. efforts in Syria, both to unseat Assad and to combat the Islamic State group. Sounding emboldened by the Russian airstrikes, Zarif said at a talk in New York that there was a difference between Russia — which was invited by 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 between 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 Assad. Likely targets are Army of Conquest insurgents who threaten Assad’s coastal strongholds from territory they have seized in the northern province of Idlib.

In the aftermath of 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 Komoyedov said his country’s “volunteers” on the way to Syria “cannot be stopped.”

As the global powers postured, gaps also deepened between local and regional participants in the war, and predictions that the Russian action would strengthen radicals in the Syrian insurgency seemed to be accurate.

Insurgent groups opposing both Assad and the Islamic State group, including some supported by the Americans, declared they would no longer participate in any peace process sponsored by Russia — which they accused of occupying their country. Some vowed to work more closely with the Nusra Front.

Forty-one insurgent factions said in a statement that Russia’s “brutal occupation has cut the road to any political solution,” the latest challenge to diplomatic efforts by a special U.N. envoy, Staffan de Mistura.

Separately, a group of prominent Saudi Arabian clerics called on Muslim and Arab countries to support a jihad, or holy war, against Assad and his Russian and Iranian patrons — even comparing the Syrian war to the Soviet Union’s invasion of Afghanistan in the 1980s and the jihad against it that drew fighters from around the world.

The statement followed a declaration from the head of the Russian Orthodox Church, Patriarch Kirill, blessing the Russian fight in Syria.

Even the Islamic State group — which appeared to be a minor participant in the events of the past week, sought to remind the outside world of its presence, destroying more antiquities. Syrian officials and local activists confirmed that Islamic State demolitionists had wrecked a renowned triple arch at the beginning of a Roman colonnade at the ancient city of Palmyra.

The potential combination of Russian ground forces and aerial attacks particularly threatens to undermine Turkey’s Syria policy, which aims for the establishment of a “safe zone” along the Turkish border where some Syrian refugees could return in the future.

Turkey and Russia have a strong bilateral relationship, but those relations have soured in recent months because of deep differences over Syria and the stalling of a pipeline project to carry Russian gas to Turkey.

U.S. Defense Secretary Ashton Carter said U.S. officials were conferring with Turkish authorities over what steps would be taken.