The attack by a shoulder-fired missile was a significant new blow in an Iraq insurgency that escalated in recent days - a "tough week," in the words of the U.S. occupation chief.
Other U.S. soldiers were reported killed Sunday in ground attacks here and elsewhere in central Iraq. The only day that saw more U.S. casualties came March 23, during the first week of the invasion that ousted Saddam Hussein.
Sunday's attacks came amid threats attributed to Saddam's party of a wave of violence against the U.S. occupation. Saturday had been planned as a "Day of Resistance" in Baghdad, though no widespread violence was reported there.
The aircraft was hit at about 9 a.m. and crashed amid cornfields near the village of Hasi, about 40 miles southwest of Baghdad and just south of Fallujah, a center of Sunni Muslim resistance to the U.S. occupation.
At the scene, villagers proudly showed off blackened pieces of wreckage to arriving reporters.
Others celebrated word of the helicopter downing, as well as a fresh attack on U.S. soldiers in Fallujah itself, where witnesses said an explosion struck one vehicle in a U.S. Army convoy at about 9 a.m. Sunday. They claimed four soldiers died, but U.S. military sources said they couldn't confirm the report.
This was a new lesson from the resistance, a lesson to the greedy aggressors," one Fallujah resident, who wouldn't give his name, said of the helicopter downing. "They'll never be safe until they get out of our country," he said of the Americans.
A U.S. military spokesman, Col. William Darley, confirmed the casualty count of 15 but said the cause of the crash was under investigation. He said witnesses reported seeing what they believed were missile trails.
"It does appear that a U.S. helicopter was probably shot down from the ground and it crashed, and a large number of Amercians, possibly 12, 13, maybe more even have died," Defense Secretary Donald H. Rumsfeld said in Washington.
Rumsfeld called it "a tragic day for America and for these young men and women. I must say, our prayers have to be with them and with their families and their loved ones.
Witnesses said they saw two missiles fired from a palm grove at the heavy transport copter. The missiles flashed toward the helicopter from behind, as usual with heat-seeking shoulder-fired missiles such as the Russian-made SA-7. The old Iraqi army had a large inventory of SA-7s, also known as Strelas.
The 10-ton Chinook - the military's heavy-list workhorse used primarily for moving troops and equipment- was the biggest U.S. target yet shot from the skies. The downed craft belonged to the Army's 12th Aviation Brigade, supporting the 82nd Airborne Division Task Force.
Insurgents have fired on U.S. aircraft before, downing two helicopters since Saddam's regime fell - though only one American was injured in those incidents. American military officials have repeatedly warned that hundreds of shoulder-fired surface-to-air missiles remain unaccounted for in Iraq since the collapse of Saddam's regime in April. The U.S.-led coalition has offered rewards of $500 apiece to Iraqis who turn the weapons in.
The death toll surpasses one of the deadliest single attacks during the Iraq war: the March 23 ambush of the 507th Maintenance Company, in which 11 soldiers were killed, nine were wounded and seven captured, including Pvt. Jessica Lynch. A total 28 Americans around the country - including the casualties from the ambush - died on that day, the deadliest for U.S. troops during the Iraq war.
The helicopter was part of a formation of two Chinooks carrying a total of more than 50 passengers to the U.S. base at the former Saddam International Airport, renamed Baghdad International Airport, which the military calls BIA.
"Our initial report is that they were being transported to BIA for R&R flights," a U.S. command spokeswoman in Baghdad said. She said at least some were coming from Camp Ridgway, believed to be an 82nd Airborne Division base in western Iraq.
Command spokesman Darley said he didn't know whether the troops were bound for leaves at home or abroad outside Iraq.
Villagers said the copters took off from the air base at Habbaniyah, about 10 miles northwest of the crash site. One villager, Thaer Ali, 21, said someone fired two missiles from the area of a date palm grove about 500 yards from where the stricken copter crashed.
Another witness, Yassin Mohamed, said he ran out of his house, a half-mile away, when he heard an explosion. "I saw the Chinook burning. I ran toward it because I wanted to help put out the fire, but couldn't get near because of American soldiers."
Witnesses said the second copter hovered over the downed craft for some minutes and then set down, apparently to try to help extinguish a fire. The downed, 84-foot-long copter was already destroyed.
At least a half-dozen Black Hawk helicopters later hovered over the area, and dozens of soldiers swarmed over the site. Injured were still being evacuated at least two hours later.
In a separate incident, the U.S. command said a soldier from the 1st Armored Division was killed just after midnight when a makeshift bomb was exploded as his vehicle passed while responding to another incident.
In Abu Ghraib, on Baghdad's western edge, U.S. troops clashed with townspeople Sunday for the second time in three days, and witnesses reported casualties among both the Americans and Iraqis. There was no immediate official confirmation.
Local Iraqis said U.S. troops arrived Sunday morning and ordered people to disperse from the marketplace and remove what the Iraqis said were religious stickers from walls. Someone then tossed a grenade at the Americans, witnesses said, and the soldiers opened fire.
The U.S. command said it had no immediate information, but Iraqi witnesses said they believed three or four Americans were killed and six to seven Iraqis were wounded.
Last Friday at the same marketplace, attempts by U.S. troops to clear market stalls from a main road led to sporadic clashes that left two Iraqis dead, 17 wounded and two U.S. soldiers wounded