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.