In the latest in months of such threats, Turkish President Recep Tayyip Erdogan announced Wednesday that Turkish forces will invade northeastern Syria “within a few days,” aiming to clear all territory held by the Kurdish YPG east of the Euphrates River. This is virtually all of the territory east of the Euphrates within Syria.

The Pentagon responded to Erdogan’s threat by saying any Turkish invasion would be “unacceptable” and a grave concern to the US military forces present in Syria. Pentagon spokesman Commander Sean Robertson warned that the war against ISIS is not over and the Kurds are a “committed partner” against them.

Kurdish officials have warned that a Turkish invasion would derail their fight against ISIS further to the south. Early in the Kurdish offensive against ISIS, they’d also had to withdraw for a time because of Turkish threats.

Turkey has repeatedly attacked Kurdish territory in Syria throughout the Syrian War, and Erdogan had previously insisted that the Kurds could hold no land west of the Euphrates. Now, he is vowing to clear out all land east of the Euphrates as well.

But Erdogan has threatened to attack this area a lot of times, without having done so before. US officials have made efforts to placate him with promises of observation posts on the border, though Erdogan is now accusing the US of building such posts to protect the “terrorists” and not Turkey.