Russian warplanes have violated Turkish airspace at least twice during the Russian air campaign against Syrian rebels, and the Secretary-General of NATO, Jens Stoltenberg, said these penetrations appear deliberate.

“It doesn’t look like an accident, and we have seen two violations of Turkish airspace over the weekend,” said Stoltenberg at a press conference Tuesday morning, as reported by the Wall Street Journal. He described these incidents as “unacceptable.”

Turkey’s president, Recep Tayyip Erdogan, was also unwilling to accept Russia’s excuses for the border violations, accusing Russia of supporting “state terrorism” by propping up Syrian dictator Bashar Assad, and suggesting Russia could lose Turkey as a “friend” with the actions it is taking.

“Some steps that we do not desire are being taken. While it won’t be fitting for Turkey to accept them, they are also counter to NATO principles, and NATO has taken a stance against them,” said Erdogan.

The Turks suggest Russian planes have done more than stray into their airspace – a Russian fighter apparently locked its radar onto a squadron of Turkish F-16s flying patrol along the Syrian border for over four minutes, while ground missile batteries inside Syria also targeted the Turkish planes.

There is some confusion about precisely who was flying the MiG-29 plane involved in the incident; some analysts believe it may have been a Syrian jet, arguing that Russia has supplied Damascus with MiG-29s in the past, but is not known to have deployed any as part of its new air force in Syria. Even if this is true, the Turks will likely hold Russia responsible for emboldening the already reckless Syrian air force.

The Wall Street Journal’s account of NATO’s emergency meeting implies there may have been other targeting-radar incidents against Turkish aircraft. While Stoltenberg stopped short of accusing Russian pilots of preparing to fire on Turkish planes, he did say that “the intelligence we have received provides me with reasons to say it doesn’t look like an accident.”

The Russians have acknowledged only one violation of Turkish airspace, which they say occurred because “adverse weather conditions” caused one of their planes to stray across the border for a few seconds.

CNN notes that at his press conference, Stoltenberg also described a “substantial buildup of Russian forces in Syria,” including “ground troops in connection with the air base they have.”