The Turkish military and forces from the Kurdish YPG have been trading fire in and around the Afrin District, in the northern Syrian Province of Aleppo, with both sides reporting an increase in military fire, and both blaming the other side for starting it.

The YPG started warning about the Afrin District on Friday, saying they’re concerned an outright Turkish attack on the district, which isn’t well defended because of the focus on the ISIS offensive, could ultimately force them to abandon the push into Raqqa, the ISIS capital, in favor of defending their territory from the Turks.

Turkish media is reporting that the YPG forces along the border opened fire on them first, and that they responded with artillery fire. Neither side is saying what casualties, if any, were suffered in this exchange, but the Syrian Observatory for Human Rights suggested both sides had been firing for more than a day, off and on.

The US hasn’t publicly responded to this particular incident, but in recent months has deployed US troops on the frontier between the two sides to try to limit the direct clashes. They are likely to try to do so again if this fight does start to interfere with the Raqqa invasion, which is clearly the top US priority right now.