Turkey has warned the US that bilateral ties will suffer if Washington adopts a bill recognising as genocide the Ottoman empire's killings of Armenians.
Armenians say many of them died in the systematic deportations
In a letter to US President George W Bush, Turkish President Abdullah Gul said "serious problems" would emerge if US lawmakers passed the bill.
The US House Foreign Affairs Committee is due to discuss the bill this week.
Ankara rejects Armenian claims that the deaths of some 1.5m Armenians in 1915-17 amounted to genocide.
It admits that many Armenians were killed, but says the deaths were a result of widespread wartime fighting in Turkey during World War I.
Armenians have campaigned for the killings to be recognised internationally as genocide. Some countries have done so.
In his letter, President Gul "drew attention to the serious problems that will emerge in bilateral relations if the bill is adopted," his office said in a statement.
Separately, Parliament Speaker Koksal Toptan warned that "it might take decades to heal negative effects" if the motion was passed.
"Then, it will be difficult to control the dynamics triggered by Turkish public reaction," Mr Toptan said in a letter to House Speaker Nancy Pelosi.
Other Turkish leaders have indicated that Ankara could consider blocking Washington's use of a key military base in Turkey that provides logistical support in Iraq and Afghanistan.
The House foreign affairs committee is expected to debate the bill on Wednesday.
If it is approved, Democratic Speaker Nancy Pelosi may allow a vote. However, it would have no binding effect on US foreign policy.
Similar bills in 2000 and 2005 were blocked by senior US politicians. Last year, Turkey cut military co-operation with France after the French parliament passed a bill to make denial of the Armenian genocide an offence - even though it never became law.