Turkey's president on Tuesday questioned the future of relations between Ankara and Moscow after multiple reports of airspace violations by Russian military planes along the Turkish border with Syria.
"Some steps that we do not desire are being taken. It is not suitable for Turkey to accept them," Recep Tayyip Erdogan said during a news conference with Belgium's prime minister in Brussels, Turkey's Hurriyet Daily News reported.
Turkey summoned Russia's ambassador for a second time Monday after what the foreign ministry said was another instance of a Russian aircraft breaching Turkish airspace.
Turkish Prime Minister Ahmet Davutoglu has warned Moscow that under Turkey's rules of engagement, its fighters are allowed to shoot down any warplane operating in Syria that violates Turkish airspace.
NATO Secretary-General Jens Stoltenberg told reporters Tuesday that the violations do not look like an accident.
"I will not speculate on the motives, I would just reiterate or restate that this is a serious violation of Turkish airspace, it should not happen again, and NATO has expressed strong solidarity with Turkey," Stoltenberg said.
He added that Russia's increased military presence in Syria raises also raises concerns because attacks have been directed not only at Islamic State fighters, but also at opposition groups, and "many civilians have lost their lives."
Marcin Zaborowski, executive vice president of the Center for European Policy Analysis, told VOA's Persian service that Russia is trying to test NATO's limits by violating Tirkish airspace. "It does seem so, that Russia is taking such steps, while Turkey is trying to stay away from the Russia-West tensions," he said.
Later on Tuesday, Russian Defense Ministry spokesman Major General Igor Konashenkov said Turkey's Defense Ministry had proposed establishing a joint working group "to coordinate activities and prevent possible incidents" related to Russian airstrikes aimed at "the destruction of the Islamic State group on the territory of Syria."
Russian planes have been conducting airstrikes in Syria since last week targeting both Islamic State fighters and what it called "terrorist" groups. Syrian President Bashar al-Assad, an ally of Russia, has frequently used the term "terrorist" to describe the rebels who have fought against his government for more than four years.
On Monday, a top Russian lawmaker said "a unit of Russian volunteers," including battle-hardened veterans who fought in eastern Ukraine, may join Syrian government troops fighting Islamic State extremists on the ground.
Admiral Vladimir Komoyedov, in comments to Russia's Interfax-AVN news agency, said Russian involvement on the ground in Syria is "likely," but he did not provide a timetable.
Remarks by Komoyedov, head of the Russian parliament's defense committee, came in response to unconfirmed media reports that Russian volunteers already have been spotted fighting alongside the Syrian army.
U.S. Defense Secretary Ash Carter on Tuesday did not confirm to reporters the involvement of Russian ground forces, but sharply criticized the idea. Calling the strategy "futile," he said Moscow would be “deepening [its] mistake in Syria" if it’s true.
Overflights called 'unprofessional'
The U.S. State Department on Monday called the intrusions by Russian jets "reckless" and provocative, and Russian military involvement in Syria "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," he said. "We'll see if it happens again."
Syria's foreign minister, Walid al-Moallem, said Russian airstrikes in his country followed months of preparations, according to the official Syrian news agency SANA on Monday.
Syrian rebels vow to fight Russian forces
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 Dorian Jones contribured to this report from Istanbul.