A Pentagon official said it was an Apache attack helicopter, which carries two crew members.
Another Apache crashed on Sunday during heavy fighting with a Shia cult near Najaf, also killing two soldiers.
Iraqi police and witnesses said the latest crash occurred about 7.30am as two Apaches were flying along a well-established air route near Taji, a major US base about 20km 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 towards 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.
Elsewhere, the US command said 18 fighters were killed in fighting on Thursday night and Friday after they opened fire on the Americans from several positions in Ramadi, 110km west of Baghdad.
No civilian or US casualties were reported, the military said.
The US forces returned fire with machine guns, tanks and finally a missile, which struck the intended target, killing at least 15 fighters, the US command said.
The fighters renewed their attacks on Friday, prompting US forces to fire another missile that killed at least three of them, the US military said.
Also in Anbar, armed men assassinated Abbas Ali Hussein, the Sunni chairman of the Fallujah city council, an outspoken critic of al-Qaida.
He was the third council chairman assassinated this year as fighters target Sunnis willing to co-operate with the US and its Iraqi partners.
In Baghdad, police said they found the bullet-riddled bodies of 23 people throughout the capital - apparent victims of Shia or Sunni death squads.
Three more bodies were found in Kut, southeast of Baghdad, and two in Mosul.
On Friday, the Iraqi government launched a verbal attack on neighbouring Syria, accusing it of making problems for Iraqi refugees while welcoming Sunni figures wanted by the Baghdad government.
"Thousands of Iraqis are being put in a difficult situation," Ali al-Dabbagh, government spokesman, told Al Hurra television.
He said Iraqis going to Syria are being given only 15-day entrance visas and some have to leave the country where it is not easy for them to enter Lebanon, Jordan or Turkey.
Al-Dabbagh added that Syrian authorities have imposed new regulations on state-run Iraqi Airways, which has not flown to Syria since Tuesday.
Syrian authorities on Wednesday denied reports arising elsewhere that it had halted Iraqi Airways flights but said it would bar Iraqi airliners lacking safety requirements or those arriving without advance permission to land.
Iraq also was angered by the Syrian president's meeting with Sheikh Harith al-Dari, the head of the predominantly Sunni Association of Muslim Scholars, who was in Damascus this week.
Iraq's interior ministry issued an arrest warrant against him in November, alleging al-Dari instigated sectarian violence. Al-Dari has said the government's bid to arrest him was illegal.
After meeting Bashar al-Assad and Farouk al-Sharaa, the vice president, on Wednesday, al-Dari said the political process in Iraq and all its achievements so far - including the constitution and the national assembly - must be abandoned as a "pre-condition for a successful reconciliation among the Iraqis".