The double violation of Turkey's airspace by Russian jets over the weekend was no accident, NATO says. Alliance Secretary General Jens Stoltenberg said Tuesday the air incursions on Saturday and Sunday "lasted a long time" and Russia "had not provided any real explanation," according to a Guardian report.

The incident comes just a week after Russian jets began bombing Syrian rebel and Islamic State group targets near the central Syrian city of Homs. Russia has been building up its forces in Syria since the beginning of September. U.S. and European officials say Russia’s only aim is to keep dictator Bashar Assad in power, not to fight the extremists also known as ISIS.

After the jets were spotted, the Turkish military scrambled F-16 jets, which escorted the Russian Sukhoi jets back into Syrian airspace.

On Monday, NATO warned Russia about entering the airspace of any of its member states in the future. “Russian military actions have reached a more dangerous level with the recent violations of Turkish airspace on 3 October and 4 October by Russian Air Force SU-30 and SU-24 aircraft in the Hatay region,” a part of Turkey bordering Syria, the North Atlantic Council, the decision-making body of NATO, stated Monday. “The aircraft in question entered Turkish airspace despite Turkish authorities’ clear, timely and repeated warnings. In accordance with NATO practice, Turkish fighter aircraft responded to these incursions by closing to identify the intruder, after which the Russian planes departed Turkish airspace.”

Russia said the airspace violation lasted just a few seconds and was due to poor weather, while U.S. Secretary of State John Kerry said Turkey, a NATO member, would have been within its rights to defend itself, according to an Agence France-Presse  report.

In response, the Turkish government summoned the Russian ambassador in Ankara for an explanation.

“The acting undersecretary of the ministry summoned the ambassador of the Russian Federation in Ankara to the ministry and strongly protested this violation, demanded that any such violation not be repeated and affirmed that, otherwise, the Russian Federation will be responsible for any undesired incident that may occur,” the Turkish Foreign Ministry said.

Despite the warning, it seems Russia may have infiltrated Turkey's airspace for a third time. An unconfirmed report by the Daily Sabah, an English-language newspaper in Turkey, tweeted Tuesday that a MiG-29 "interfered" with up to eight Turkish F-16 jets patrolling the Syrian border on Monday.