Ratcheting up the confrontation over the Syria war, Russia said Monday, October 5, 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 New York Time reports.

Russia squared off with Turkey and its NATO allies, calling the air incursion an innocent mistake because of foul weather — a claim American officials rejected.

A second airspace violation was committed on Sunday.

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 was disclosed Monday by his top military liaison to the Parliament, Adm. Vladimir Komoyedov.

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.