NATO offered Monday to help Turkey defend its southern border with Syria after Russian jets infiltrated the country’s sovereign air space earlier this month. Secretary-General Jens Stoltenberg made the offer during a Parliamentary Assembly meeting of NATO member country legislators in Stavanger, Norway.

"Turkey is a strong ally and they have the second-largest army," Stoltenberg told Reuters. "They have a capable air force, so the Turkish armed forces are the first responders, but NATO is there to help and assist them if they need."

Russian violations of Turkey’s airspace came amid increased military activity on the Syria-Turkey border where Russian jets are conducting airstrikes and other operations against the Islamic State group and Syrian rebel groups opposed to Syrian leader Bashar Assad’s authoritarian regime. The Kremlin had been laying the groundwork for the strikes as it began building up a military force more than six weeks ago. Along with troops and heavy weaponry being ferried into the country on ships that docked at an old Soviet base on Syria’s Mediterranean coast, military aircraft had also been landing in the government-held north of the country.

Russia claimed the violations, which took place Oct. 3-4, were because of a navigational error. Stoltenberg and Turkish President Recep Tayyip Erdoğan both doubted it was an error, with Erdoğan issuing Moscow a warning its behavior could threaten economic and diplomatic relations between the two countries.

“Our positive relationship with Russia is known. But if Russia loses a friend like Turkey, with whom it has been cooperating on many issues, it will lose a lot, and it should know that,” Erdoğan told reporters in Brussels last week,  Reuters reported.

The Syrian civil war has been raging for more than 4 1/2 years. In that time, around 250,000 have been killed and 4 million displaced.