Washington has supported and encouraged the Kurdish parties of the area in their fight against a common foe, the Islamic State jihadist group.
But the State Department said Wednesday it would not support the breakup of the country and that any new federal model would have to emerge from peace talks.
"We've been very clear that we won't recognize any self-rule autonomous zones within Syria," spokesman Mark Toner told reporters.
"This is something that needs to be discussed and agreed upon by the relevant parties in Geneva and then by the Syrian people themselves."
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Representatives of Bashar al-Assad's government and of the opposition ranged against him are negotiating an end to the civil war under UN auspices in Geneva.
But parties representing Syria's Kurdish minority have not been invited to the talks and are instead trying to create a unified region of their own.
Any bid to expand an existing system of self-rule will anger Turkey, wary of anything that might encourage Kurdish separatism within its own borders.
More than 150 delegates from Kurdish, Arab, Assyrian and other parties met Wednesday in Rmeilan, in Syria's northeastern Hasakeh province.
The meeting adjourned in the early evening and will reconvene on Thursday, when a decision on declaring a semi-autonomous northern region will be announced.