"We presume that all were lost," the official, who asked not to be identified, told Reuters on Wednesday when asked if all the soldiers had apparently been killed in the crash.
Earlier in the day, the US army said the military helicopter may have been downed by enemy fire, and the fate of the 17 men on board was still unknown.
The Taliban have said they brought down the aircraft.
The CH-47 Chinook crashed west of Asadabad in Afghanistan's Kunar province bordering Pakistan. Afghan officials said it was hit by a rocket, which the Taliban claimed to have fired.
Sources told Aljazeera's Afghanistan correspondent Wali Allah Shahin that the Taliban fired rocket-propelled grenades at the helicopter, causing it to crash.
Site sealed off
Witnesses in the area confirmed to Aljazeera they saw rocket-propelled grenades being fired at the helicopter, but were not sure the grenades had directly hit it.
The US statement said US and Afghan troops had sealed off the crash site to block any enemy movement towards or away from it and US aircraft were flying overhead.
The Taliban claimed responsibility
for the helicopter crash
"This is a tragic event for all of us, and our hearts and prayers go out to the families, loved ones and service members still fighting in the area," US Brigadier-General Greg Champion said in the statement. "Our courage and commitment to America's fight in the global war on terror will not waver," he said.
"This incident will only further our resolve to defeat the enemies of peace."
A Taliban member called Muhammad Ismail, reported four other US soldiers had been killed in a separate attack, Shahin said.
Operation Red Wing
Ismail said that the crashed helicopter was on its way to help the four soldiers and transfer reinforcements when it was shot down.
The US statement said the helicopter had been transporting forces into the area as part of Operation Red Wing, a continuing effort aimed at defeating al-Qaida fighters in Kunar.
It said recent activity in the area had been described as a series of harassing attacks and intelligence-gathering activities against Afghan and US forces.
The crash was the second of a US Chinook in Afghanistan in less than three months.
The other helicopter crashed in a dust storm in Ghazni province southwest of the capital, Kabul, while on a routine mission on 6 April, killing 18 people, including 15 military personnel.
It was the deadliest military air accident since Washington first deployed troops to the country in 2001.