The US and its allies are already uneasy about Russia’s air strikes on Bashar al-Assad’s enemies in Syria—especially since Moscow has targeted groups supported by the US-led coalition. Now it appears that the Russian planes conducting those strikes have been taking detours into Turkish air space—a sign that Moscow is not willing to abide by no-fly zone rules.

Turkey says two of its F-16 fighter jets intercepted a Russian war plane on Turkey’s side of the Syrian border on Saturday (Oct. 3), and that on Sunday (Oct. 4), two of its jets were “harassed” by an unidentified MiG-29 plane for more than five minutes while flying near the Syrian border. NATO confirmed these incidents but said the unidentified plane involved in Sunday’s incident was a Russian SU-30 or SU-24, rather than a MIG

The Turkish military also said that on Monday (Oct. 5), an unidentified plane locked its radar on up to eight Turkish jets and that missile systems inside Syria were “locked on” to Turkish planes for several minutes, as well.

Turkey and Russia are on opposing sides of the Syrian conflict: Turkey wants Assad ousted, and Russia wants to prop him up. US state secretary John Kerry said he is “greatly concerned” about the recent Russian incursion into Turkish air space. According to an anonymous NATO official quoted by the Wall Street Journal: “It was a political statement from the Russians that said, ‘We will fly anywhere we want.’”