BAGHDAD, Iraq — A Sea Knight helicopter went down northwest of Baghdad on Wednesday, the military said, the fifth helicopter lost in Iraq in just over two weeks.

The CH-46 helicopter went down about 20 miles northwest of the capital, U.S. military spokesman Maj. Gen. William Caldwell said, but he declined to comment on casualties.

“A quick reaction force is on site and the investigation is going on as we speak,” he said in Baghdad, without giving a cause for the crash. “It would probably be inappropriate for me to talk about whether or not there are or are not casualties.”

Witnesses said the helicopter had been shot down in a field in the Sheik Amir area northwest of Baghdad, sending smoke rising from the scene, in a Sunni-dominated area between the Taji air base, 12 miles north of Baghdad, and Karmah, 50 miles west of the capital.

“The helicopter was flying and passed over us, then we heard the firing of a missile,” said Mohammad al-Janabi, a farmer who was speaking less than a half-mile from the wreckage. “The helicopter then turned into a ball of fire. It flew in a circle twice, then it went down.”

The helicopter went down five days after a U.S. Army helicopter crashed in a hail of gunfire north of Baghdad, police and witnesses said. The U.S. command said two crew members were killed in that crash, and the al-Qaida-affiliated group the Islamic State of Iraq claimed responsibility.

Three other helicopters also have gone down since Jan. 20, killing a total of 19 Americans — 14 troops and five civilian security contractors.

The twin-rotor CH-46 is used by the Marines primarily as a cargo and troop transport, and can carry 25 combat-loaded troops, according to the think tank GlobalSecurity.org.

U.S. helicopter crashes in Iraq since August 2006:

_ Feb. 7: A CH-46 Sea Knight helicopter went down in a Sunni-dominated area northwest of Baghdad, but there was no initial word on casualties.

_ Jan. 28: An AH-64 Apache helicopter went down during heavy fighting near Najaf, south of Baghdad, killing the two crew members.

_ Jan. 23: An OH-6A helicopter owned by the private security company Blackwater USA crashed in Baghdad in heavy gunfire in an incident that left five civilian contractors dead.

_ Jan. 20: A Black Hawk helicopter crashed in Diyala province northeast of Baghdad, killing 12 soldiers aboard. The military said it might have been shot down by a shoulder-fired weapon, although their investigation was continuing.

_ Dec. 11, 2006: A Marine helicopter made hard landing, injuring 18 on board. The military said it was not hit by gunfire.

_ Dec. 3: A Sea Knight helicopter carrying 16 Marines went down in a lake in Anbar, killing four.

_ Nov. 6: A helicopter crashed in Salahuddin province, north of Baghdad, killing the two soldiers on board. No gunfire was reported in the area at the time of the crash.

_ Aug. 8: A Black Hawk helicopter crashed into a body of water, in an apparent accident, killing two soldiers.

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From the Debka files in May of 2006:

In the past two weeks, Iran has been pumping into Iraq two types of extra-lethal weapons in very large quantities. They have already taken their toll in the shooting down of two military helicopters – one American and one British – and an estimated 19 deaths of US military personnel.

DEBKAfile’s military and intelligence sources estimate the delivery to Iraqi insurgents as consisting of around 1,000 SA-7 Strela ground-air missiles made in Iran, and a very large quantity of a newly-developed roadside bomb, loaded with compressed gas instead of ball bearings and cartridges, to magnify their blast and explosive power.

The supplies have been distributed across Iraq – Basra and Amara in the south, Baghdad and its environs, Haditha in the west, and Mosul in the north.

The new bombs, developed jointly by Iran’s Revolutionary Guards and the Lebanese Hizballah, have already gone into service with the Shiite terrorists on the Lebanese border with Israel. Israeli military sources say it is only a matter of time before the deadly roadside bombs, already used in Iraq, will also reach Palestinian areas in the West Bank and Gaza Strip.

In Iraq, the new weaponry has had three major effects:

1. The guerrilla-terrorist groups which received the shoulder-carried, highly mobile Strela missiles have scored three hits in fourteen days. On May 6, they fired a missile from one of Basra’s crowded alleys and downed a British military helicopter, killing all four military personnel aboard. Sunday, May 14, Iraqi insurgents shot down an American helicopter, killing its two crewmen over Yussifiya, inside the Triangle of Death south of Baghdad.

2. The number of roadside bomb attacks, their precision and lethality is going up all the time. Sunday, May 14, four US soldiers died in these blasts in the western Anbar province and Baghdad, while 2 British soldiers were killed and another injured at the same time near Basra. In seven days, the British force stationed in southern Iraq lost seven men, a record for that space of time in the three-year war. In the first half of May, US troop losses spiraled to 19, most of them the victims of the new roadside bombs.

3. Together with the new Iranian weapons, a new array of Shiite terrorist groups has sprung up and is hitting American and British troops. The coalition has imposed a blackout on this disturbing development.

Until now, the insurgent forces fighting the coalition consisted mostly of Baathists, Islamist and al Qaeda. The only Shiite enemy was the radical Mogtada Sadr and his Mahdi Army. The appearance of the new Shiite insurgents is a dread milestone in the Iraq war, one which has caught US and UK commanders by surprise and unprepared for the steep rise in troop losses.

DEBKAfile’s Exclusive Iraq sources offer some information on the new groups. One is located north of Baghdad and calls itself Brigades of the Imam Kazim. Another, called Brigades of Imam Ali, claimed the attack on April 27 in Nasiriya in which one of their new roadside bombs killed two Italian troops. In the Rostumiya region south of Baghdad, a Shiite group called Brigades of the Imam Hadi has begun operating. Our sources report that this group has been firing Katyusha rockets at American bases in the region, similar to the mortar attack directed at a British base in Amara Monday, May 15.

After each attack, these unknown quantities issue bulletins describing their actions, some accompanied by video footage from the scene of action.

The blackout was imposed on the new Shiite groups in the absence of American or British intelligence on who they and their commanders are, how they operate and what makes them tick. Research must start from square one to find out whether they are being controlled from Tehran, some Iraqi Shiite faction or elements which chanced to lay hands on the new-fangled weaponry.