As Turkey and the United States continue to spar over the imminent arrival of Russian made S-400 missiles on Turkish soil, and more recently over Turkish moves in the eastern Mediterranean, a long-running quarrel over US support for Kurdish militants in Syria seems to have been put on the back burner — for now.

It may be that the NATO allies are so seized with the S-400s — with the United States threatening to unleash an array of sanctions and Turkey refusing to blink — that there is little room to squabble over Syria. More likely it is because their efforts to agree on a proposed safe zone along the Syria-Turkey border have hit a wall.

Jim Jeffrey, the Donald Trump administration’s top policy official on Syria, acknowledged the lack of progress without characterizing it as such in a brief but candid interview with the German Marshall Fund that was published this week.

Jeffrey made several things clear, none of which will please Turkey. For one, he stated, albeit elliptically, that US forces in northeastern Syria are staying indefinitely. Touching on Trump’s shock announcement in December that he had ordered the withdrawal of all troops from Syria, Jeffrey said, "It’s been reversed in the sense that, while the president is continuing with the withdrawal, his plan is to keep for an indeterminate time, a residual force in the northeast.”