A top aide to Turkish President Recep Tayyip Erdogan hinted yesterday that United States commandos in northern Syria could be targeted by Turkish rockets if they continue to work with Kurdish fighters along the border with Turkey.

The comments by Ilnur Cevik, a senior political advisor to Erdogan, came the same day the President sat down with Russian President Vladimir Putin to discuss plans for "safe zones" meant to stop the violence in Syria.

And they come two weeks before Erdogan visits Washington for his first meeting with President Donald Trump.

If the Kurds and Americans continue to work together, Cevik said during a radio interview, "we won't be considering the fact that there are armoured American vehicles ... All of a sudden, by accident, a few rockets can hit them".

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The veiled threats for a Nato ally to potentially use force against US troops highlight just how risky and complicated US intervention in the Syrian civil war has become, and could further sour relations between Washington and Ankara that have already been strained following a failed coup last year in Turkey.

For weeks, US Army Special Forces troops have been a visible presence in Kurdish areas of northern Syria, first showing up in the town of Manbij which was being threatened by Turkish-backed Syrian Arab fighters.

Over the weekend, Americans troops were again photographed driving through the town of Qamishli, near the site of last week's Turkish airstrikes which killed 18 US-backed Kurdish militias.

The US show of force is a very public reminder of American support for the Kurds, who make up a majority of the Syrian Democratic Forces, a 50,000-strong collection of local militias currently moving on the Isis (Islamic State) stronghold of Raqqa.

US military commanders say the Kurds are the only viable military option to defeat Isis on the ground in Syria.