An al Qaeda-affiliated group claimed responsibility and said its fighters had "new ways" to attack American planes.
A brief U.S. military statement gave no reason for the crash and did not identify the type of aircraft. A Pentagon official said it was an Apache attack helicopter, which carries two crew members.
The official spoke on condition of anonymity because he was not authorized to release the information. Another Apache crashed Sunday during heavy fighting with a Shiite cult near Najaf, also killing two soldiers.
Iraqi police and witnesses said the latest crash occurred about 7:30 a.m. as two Apaches were flying along a well-established air route near Taji, a major U.S. base about 12 miles north of Baghdad.
One helicopter was struck by heavy machine gunfire but continued flying, the witnesses said. The other helicopter banked sharply and flew back toward the source of fire, apparently to attack the target.
But that helicopter was also struck by ground fire, exploded in a ball of fire and crashed, the witnesses said. The other helicopter flew away, they said. The witnesses spoke on condition of anonymity, fearing for their own safety.
The United States has lost more than 50 helicopters in Iraq since May 2003, about half of them to hostile fire.
However, the loss of four helicopters since Jan. 20 has raised new questions about whether Iraqi insurgents are using more sophisticated weapons or whether U.S. tactics need changing.
Three of the latest crashes involved Army helicopters — two Apaches and one Black Hawk. The fourth was an OH-6A observation helicopter operated by the Blackwater USA security firm. All were believed shot down, and 20 Americans, including four civilian, died in the crashes.
In Washington, Gen. Peter Pace, chairman of the Joint Chiefs of Staff, acknowledged that insurgent ground fire in Iraq "has been more effective against our helicopters in the last couple of weeks."
But Pace said it was unclear whether "this is some kind of new tactics or techniques that we need to adjust to."
The Islamic State of Iraq, an al Qaeda-linked group, claimed on Friday that it shot down the Apache near Taji in a statement posted on an extremist Web site.
"We tell the enemies of God that the airspace of the Islamic State in Iraq is prohibited to your aircraft just like its lands are," the statement said. "God has granted new ways for the soldiers of the State of Iraq to confront your aircraft."
Iraqi insurgents have used heavy machine guns, rocket-propelled grenades and shouldered-fired SA-7 anti-aircraft missiles throughout the Iraq conflict. U.S. officials believe Iran is supplying Shiite militias with new weapons including more powerful roadside bombs, Katysuha rockets and a newer class of rocket-propelled grenades.
Some of those weapons could have found their way into the hands of Sunni insurgents, who operate around Taji.
The U.S. military relies heavily on helicopters to avoid roadside bombs and insurgent ambushes. Any new threat to helicopters would be a serious challenge to the military as it gears up for a major crackdown against Sunni insurgents and Shiite militias in Baghdad.
Helicopters are always vulnerable to ground fire, said Stephen Trimble, Americas bureau chief for Jane's Defence Weekly. "A well-placed bullet can pretty much take down any helicopter," he said.
Protecting helicopters from attack is significantly more complicated than defending against roadside bombs, Trimble said.
"What you would do with a Humvee is up armor it," he said. But helicopters can't support a significant increase in weight. The U.S. military is looking into technology that tracks and fires at rocket-propelled grenades, he said, but its use on helicopters is a long way off.
Apaches carry multiple high-tech defenses, including long-range sensors, radar jammers and an infrared jammer for countering incoming missiles.
However, the Apaches, which were designed to fight the Soviet Union on the plains of central Europe, have proven vulnerable to intense ground fire.
During the 2003 invasion of Iraq, more than 30 Apaches had to break off an attack after suffering heavy damage in fighting with the Iraqi Republican Guard. One helicopter crashed but the two crew members survived.
Elsewhere, the U.S. command said 18 insurgents were killed in fighting Thursday night and Friday after insurgents opened fire on the Americans from several positions in Ramadi, 70 miles west of Baghdad. No civilian or U.S. casualties were reported, the military said.
Ramadi, the capital of the western province of Anbar where Sunni insurgents remain well-entrenched, has seen some of the bloodiest street battles of the war.
The U.S. forces returned fire with machine guns, tanks and finally a missile, which struck the intended target, killing at least 15 insurgents.
Insurgents renewed their attacks on Friday, prompting U.S. forces to fire another missile that killed at least three attackers.
Also in Anbar, gunmen assassinated the Sunni chairman of the Fallujah City Council, Abbas Ali Hussein, an outspoken critic of al Qaeda. He was the third council chairman assassinated in Fallujah this year as insurgents target Sunnis willing to cooperate with the U.S. and its Iraqi partners.
The U.S. military said six more troops died Thursday, two in fighting in Anbar province, one of an apparent heart attack and three in vehicle accidents.
The deaths raise to at least 3,092 the number of members of the U.S. military who have died since the Iraq war started in March 2003, according to an Associated Press count. At least 2,480 died as a result of hostile action, according to the military's numbers.
In Baghdad, police said they found the bullet-riddled bodies of 23 people throughout the capital — apparent victims of Shiite or Sunni death squads. Three more bodies were found in Kut, southeast of Baghdad, and two in Mosul.
Prime Minister Nouri al-Maliki met Friday with U.S. Ambassador Zalmay Khalilzad and the outgoing U.S. commander in Baghdad, Gen. George Casey, to discuss a planned major military operation against armed groups in Baghdad, state television said. It gave no further details.
Iraqi officials in Hillah, a Shiite city about 60 miles south of Baghdad, announced a three-day mourning period after a devastating suicide attack Thursday. Police spokesman Capt. Muthanna Khaled said at least 73 people were killed and 163 wounded.
Police and witnesses said two bombers strolled into the central Maktabat market about 6 p.m. when the area was packed with shoppers buying food for the evening meal.
Police said an officer, Ahmed Abed Majood, became suspicious and grabbed one of the bombers but he managed to detonate his explosives. His partner then blew himself up, too.
No group claimed responsibility, but many residents blamed Sunni insurgents. The Shiite city, located in a religiously mixed province, was the scene of one of Iraq's deadliest attacks — a February 2005 suicide car bombing that killed 125 people.