BEIRUT, Lebanon — Feeling betrayed by the United States, its Kurdish allies in Syria asked the Syrian government on Friday to protect them from possible attack by Turkey.

The request surprised some American officials and could help open the way for the forces of President Bashar al-Assad of Syria, backed by Russia and Iran, to start retaking the Kurdish-held part of the country near Turkey’s border.

That would be a big step toward Mr. Assad’s goal of reclaiming all of Syria, upended by almost eight years of war.

It was also the first sign that President Trump’s abrupt announcement last week that he was withdrawing American troops from Syria was not only shifting alliances in the conflict but directly benefiting Mr. Assad — a brutal autocrat once described by Mr. Trump as an “animal” responsible for chemical attacks and other atrocities.

American-backed Kurdish People’s Protection Units, or Y.P.G., said the Syrian government should send troops to the city of Manbij, near the Turkish border.

The request amounted to a United States ally calling on an enemy of the United States to protect it from another American ally, Turkey.

The Kurdish militias are regarded by Turkey as dangerous, autonomy-minded insurgents. The United States regards them as valuable partners in helping rout Islamic State extremists from Syria — the original purpose of the American military deployment four years ago.

Although the American troops in Syria number only about 2,000, they have been a deterrent to an assault on the Kurdish militias by the Turks. The American presence also discouraged Mr. Assad’s forces from sweeping into the area even as they retook major areas elsewhere from anti-government fighters, often with the support of Russia and Iran.

With the request for help on Friday, the Kurds invited Mr. Assad into at least some of those areas that he had coveted.