A bill recognising the killing of Armenians in Ottoman times as genocide has cleared its first hurdle in the US Congress despite Turkish warnings.
Armenians want Turkey to admit the mass killings amounted to genocide
It passed through the House Foreign Affairs Committee by 27 votes to 21 - the first step towards holding a vote in the House of Representatives.
Turkish President Abdullah Gul reacted swiftly to the result, saying the move was "unacceptable" and had no validity.
President George W Bush had urged the committee not to approve the bill.
"Its passage would do great harm to our relations with a key ally in Nato and in the global war on terror," the US president said.
Turkey is a regional operational hub for the US military, and some suggest access to Incirlik airbase, or other supply lines crucial to US forces in Iraq or Afghanistan, could be cut in response.
After the vote, the US Undersecretary of State, Nicholas Burns, told the BBC that the Bush administration was "deeply disappointed".
"The United States recognizes the immense suffering of the Armenian people due to mass killings and forced deportations at the end of the Ottoman Empire," he said.
"We support a full and fair accounting of the atrocities that befell as many as 1.5m Armenians during World War I, which House Resolution 106 does not do."
The Turkish president also attacked the measure, saying some politicians had "closed their ears to calls to be reasonable and once again sought to sacrifice big problems for small domestic political games".
"This unacceptable decision of the committee, like similar ones in the past, is not regarded by the Turkish people as valid or of any value," Mr Gul said, according to the Anatolian news agency.
The BBC's Sarah Rainsford in Istanbul says it is very unusual to hear such high-level political reaction so late at night - a sign of how seriously it takes this.
This resolution is largely symbolic and is non-binding, but that will make little difference to most Turks, our correspondent says.
Turkish politicians will now focus on trying to keep the resolution from a final vote on the House floor, she adds.
Correspondents say Wednesday's result means that only a change of heart by the opposition Democrats, who control Congress, can now stop a full vote on the bill.
A German soldier took photos of Armenian deportees at the time
Divisions within the Foreign Affairs Committee crossed party lines with eight Democrats voting against the measure and eight Republicans voting for it.
Tom Lantos, the committee's chairman, had opened the debate by admitting the resolution posed a "sobering" choice.
"We have to weigh the desire to express our solidarity with the Armenian people... against the risk that it could cause young men and women in the uniform of the United States armed services to pay an even heavier price than they are currently paying," he said.
Mr Lantos, himself a survivor of the Jewish Holocaust, said he would introduce a resolution praising US-Turkish friendship next week, according to AFP news agency.
The Senate Foreign Relations Committee is expected to take up their version of the resolution in the future.