A top Russian lawmaker said "a unit of Russian volunteers," including battle-hardened veterans who have fought in eastern Ukraine, may join Syrian government troops fighting Islamic State extremists and other terrorists in that country.
Admiral Vladimir Komoyedov, in comments to Russia's Interfax-AVN news agency Monday, said Russian involvement on the ground in Syria is "likely."
Komoyedov, the head of the Russian parliament's defense committee, did not suggest a timetable for any Russian involvement, and there was no immediate comment from the Kremlin by late Monday.
The lawmaker's remarks came in response to unconfirmed media reports that Russian volunteers already have been spotted fighting alongside the Syrian army.
NATO seeks answers
Separately, NATO demanded an immediate explanation after Turkish reports of incursions over the past two days by Russian fighter jets targeting what Moscow said were terrorist targets in northern Syria. A top Moscow official blamed poor weather for the overflight.
Russia last week began airstrikes against targets in Syria, claiming attacks on both Islamic State positions and what it called "terrorist" groups — rebel factions that oppose the Syrian government.
WATCH: NATO Secretary General Jens Stoltenberg on 'violations'
NATO Secretary General Jens Stoltenberg said Monday that he met with Turkish Foreign Minister Feridun Sinirlioglu to discuss Russian operations in Syria.
The organization's decision-making body, the North Atlantic Council, said Turkish authorities repeatedly warned multiple Russian planes to leave the airspace. It called the incursions "irresponsible behavior" and asked for Russia to explain what happened.
Russian media quote Defense Ministry spokesman Major General Igor Konashenkov as saying the incident involving the Russian aircraft on the Syrian-Turkish border occurred as a result of “adverse weather conditions in the area,” and that there is no reason to look for “conspiratorial reasons."
He also said the commanders of the Russian aircraft group in Syria have “taken the necessary measures to prevent similar incidents in the future.”
Western governments have criticized Russia's involvement in Syria, saying Russian strikes are targeting rebels instead of extremists, while NATO chief Stoltenberg says they are "not contributing to the security and stability of the region."
WATCH: Russian Foreign Ministry posts video of daylight flights in Syria
The U.S. State Department on Monday called the intrusions "reckless" and provocative, and Russian military involvement "a strategic mistake, in general."
The White House also voiced concern, while an unnamed senior U.S. defense official said the intrusions were not accidental. "This was the kind of unprofessional conduct we were hoping to avoid," the official said.
Michael O'Hanlon, a senior analyst at the Brookings Institution, told VOA that, for the moment, he would offer Russia "the benefit of the doubt and assume [the overflights] were a mistake. I don't know that Russia was trying to provoke anybody.
"I could be wrong," O'Hanlon said. "We'll see if it happens again."
Syria Foreign Minister Walid al-Moallem said Russian airstrikes in Syria followed months of preparations, according to the official Syrian news agency SANA on Monday.
Moscow's latest airstrikes
The Russian Defense Ministry said Monday that its forces struck nine Islamic State targets during the past day in Hama, Homs, Latakia and Idlib provinces, hitting artillery, communications and command centers along with a training camp
Foreign Minister Sergei Lavrov said Iraq, where the United States has been leading coalition airstrikes against the Islamic State group, has not asked Russia to carry out strikes there.
Lavrov also said Russia is open to establishing contact with the Free Syrian Army, one of the rebel groups that has been fighting the government of Syrian President Bashar al-Assad for the past four years.?
Rebels vow retaliation
Dozens of Syrian insurgent groups on Monday, however, vowed to attack Russian forces in the wake of Moscow's air campaign in the country.
"This new reality requires the region's countries and the allies in specific to hasten in forming a regional alliance to face the Russian-Iranian alliance that occupies Syria," 41 factions said in a statement released by the Ahrar al-Sham group.
Assad said Sunday the entire Middle East would be destroyed if Russia's aerial bombardment of militants opposed to his government does not succeed.
VOA's Jonas Bernstein contributed to this report from Washington.