BAGHDAD, Iraq (CNN) -- All 13 people aboard a U.S. military helicopter were killed when it went down northeast of Baghdad on Saturday, the U.S. military said.

The victims were described as passengers and crew members. Their identities were "being withheld pending notification of next of kin," the military said in a statement.

The military did not disclose the crash site and did not yet know what caused the copter to crash.

"Emergency coalition forces responded and secured the scene," the military said in the statement.

Last month, a CH-53 helicopter crashed shortly after takeoff in Anbar province, killing a U.S. Marine.

Two years ago, a U.S. Marine Corps helicopter crashed in western Iraq, killing 31 Marines. Officials believe bad weather was the culprit.

Soldiers, Marine killed

Also on Saturday, the military announced that two U.S. soldiers and a Marine were killed in separate incidents in Iraq.

A roadside bomb killed a Task Force-Baghdad soldier and wounded another Saturday in northern Baghdad.

A Task Force Lightning soldier in Tikrit died Friday when a roadside bomb exploded, and a U.S. Marine was killed in combat Friday in Anbar province.

The deaths bring the U.S. military death toll to 3,024 -- and 27 for the month of January. Seven military contractors also have been killed since the U.S. invasion.

Military: Raids target al Qaeda

U.S.-led coalition forces arrested 25 terrorist suspects, including the driver of a senior al Qaeda leader, according to the U.S. military.

The raids in western Baghdad and the cities of Tarmiya, Falluja and Balad were targeting al Qaeda members and terrorists involved in suicide attacks and the smuggling of foreign fighters into the country, the military said.

No U.S. or coalition troops were hurt, the military reported.

Mortars mar start of Shiite holy month

Several mortar rounds exploded Saturday outside the provincial government complex in Karbala, despite tight security there because of the start of the Shiite holy month of Muharram, an Interior Ministry spokesman said.

No casualties were reported, Brig. Gen. Abdul Kareem Khalaf told CNN, adding that the rounds had struck the perimeter of the complex.

Shiite pilgrims have been flocking to the city, south of Baghdad, to take part in the rites of Ashura, which begins in 10 days, and to commemorate the martyrdom of Hussein, the grandson of the Prophet Mohammed.

Karbala has two of the holiest shrines for Shiites -- the Imam Hussein and Imam Abu Fadhel al-Abbas -- and attacks have disrupted Shiite religious occasions in the past, Khalaf said.

CNN's Sam Dagher contributed to this report.