The US House of Representatives Foreign Affairs Committee has frayed Turkish tempers.
It approved a resolution calling the 1915 mass killings of Armenians genocide, in a first step towards holding a full vote in the House.
Congress is controlled by opposition Democrats.
The Bush administration has expressed its disappointment.
US Defence Secretary Robert Gates spoke from London, where he was meeting his British counterpart.
“I am quite concerned about the Armenian genocide resolution,” he said.
“The problem that we have is that this is clearly a very sensitive subject for one of our closest allies and an ally that is incredibly important to the United States in terms of our operations in Iraq.”
Turkey plays a crucial role in the supply chain to US troops in Iraq. But Iraq could also be where Ankara might express its anger at the American vote.
Turkey wants to send its troops over the border, to root out Kurdish rebels who use Iraq as their base.
The US objects to such a move but, after the genocide vote, diplomats believe Turkey will be far less likely to heed the wishes of Washington.