Turkey summoned Russia's ambassador for a second time Monday after what the foreign ministry said was a second instance of a Russian plane violating Turkish airspace near the Syrian border.
Turkey first lodged its protest with the Russian ambassador and warned of consequences of any future incursions after a Russian jet crossed into Turkish territory on Saturday. The foreign ministry said the second airspace violation took place on Sunday.
Russia's defense ministry said Monday that incident was the result of bad weather conditions in the area, and that there was no reason to look for conspiracy theories.
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 concerns because it has attacked not only Islamic State fighters, but also other opposition groups and "many civilians have lost their lives."
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.