Two U.S. soldiers were killed when their helicopter was shot down and crashed in central Iraq, the military said Friday, and the government prepared to ring Baghdad with tens of thousands of security forces to curb the rampant insurgency.

The two-seat OH-58 Kiowa (search) reconnaissance helicopter was supporting combat operations when it was shot down Thursday by small-arms fire near Buhriz, a former Saddam Hussein stronghold about 35 miles north of Baghdad, the military said. Another returned to base safely despite being hit, the military said.

Both dead soldiers were with Task Force Liberty, under the command of the Tikrit-based 42nd Infantry Division.

At least 1,652 members of the U.S. military have died since the beginning of the Iraq war in 2003, according to an Associated Press count.

In other violence Friday, an ambush targeting a police patrol in the northern city of Mosul (search) killed a policeman and a bystander caught in the crossfire, said police Brig. Saeed Ahmed.

An intended car bomb attack against a police patrol instead killed two civilians and wounded 18 others Friday in Tikrit (search), 80 miles north of Baghdad, said police Lt. Khudhair Ali. Six policemen were among the wounded, said Dr. Mohammed al-Zubaidi, a hospital official.

In northern Baghdad, a guard at an Azamiyah carpentry factory was killed when four mortar rounds landed on the building, said police 1st Lt. Sadoun Abdul Ridha. Four others were wounded.

Insurgents destroyed several sections of an oil pipeline on Baghdad's western outskirts, near Abu Ghraib. Video taken by Associated Press Television News showed billowing black clouds of smoke.

The violence came as Iraqi authorities prepared the largest show of force in the capital since Saddam's ouster. Increasing militant attacks have killed more than 650 people since the country's new government was announced April 28.

More than 40,000 Iraqi police and soldiers, backed by American troops and air support, will set up hundreds of checkpoints, raid houses and search vehicles as part of "Operation Lightning," Iraqi's interior and defense ministers announced Thursday.

An Internet statement posted in the name of the Al Qaeda in Iraq (search) terrorist group challenged Iraqi authorities, saying the crackdown won't deter them from "fighting for God."

The statement, which could not be immediately authenticated, apparently referred to Interior Minister Bayan Jabr and Defense Minister Saadoun al-Duleimi, describing them as "two unbeliever apostates, followers of the worshippers of the cross, (who) have announced they are deploying 40,000 cowards and apostates in Baghdad."

Authorities did not say how long Operation Lightning would last, and it was uncertain whether the Iraq security forces are capable of mounting a sustained campaign.

Iraq has 89,400 security personnel, including commando units, in the Interior Ministry, according to the U.S. military. The figure may include some deserters. Another 75,800 forces are in the Iraqi military, mostly the army.

Baghdad will be divided into two sectors, Karkh on the west bank of the Tigris River that separates the city, and Risafa on the east. Karkh will be split into 15 sub-districts and Risafa into seven. Security forces will operate 24 hours a day.

Northwest of Baghdad, in the city of Haditha (search), more than 1,000 U.S. troops continued a sweep for insurgents responsible for attacks against coalition troops, ordering at least one airstrike Thursday against a suspected militant position. At least 11 insurgents and one Marine have been killed since the mission began Wednesday.

Some insurgents in Haditha are believed loyal to Iraq's most-wanted militant, Abu Musab al-Zarqawi (search), whose fate has been the subject of intense speculation this week.

Jabr and al-Duleimi said Thursday the Jordanian-born leader of Al Qaeda in Iraq had been wounded, confirming several Internet statements making the same claim this week. An Internet statement also claimed the group had appointed a deputy to fill in for al-Zarqawi. But a rival statement rejected the claim.

American authorities, meanwhile, are investigating the shooting death early Friday of a detainee at the U.S.-run Abu Ghraib prison, a military statement said. It did not elaborate.

Another U.S. inquiry has been launched into the killings Thursday of three Iraqis, who were in a van that was fired on by U.S. soldiers in southeastern Baghdad, Master Sgt. Greg Kaufman said.

Soldiers shot at the van after its driver failed to respond to warnings to stop, the spokesman said. Soldiers had seen several Iraqis flee in the vehicle after stopping near a car tire on the road, raising suspicions they may have been planting a bomb. No explosives were found.

Police Sgt. Najim Aboud said two of the slain Iraqis were brothers, and their mother also was in the van.

In other developments, police announced breakthroughs in three car bomb attacks carried out this month, including one Thursday in Baghdad's northern Shula suburb that killed five people.

Police raided two Baghdad homes overnight and detained four suspects, including three brothers, in the Shula bombing, which also wounded 17 people, said army Staff Brig. Sadiq Jaafar.

Police paraded three men, including two brothers, who confessed to carrying out a car bombing Monday outside an eastern Baghdad restaurant popular with police that killed at least eight people.

Wasit police commander Maj. Gen. Abdul al Ameer said several people were detained Friday in a May 6 car bombing at a market in Suwayrah, 25 miles south of Baghdad, that killed 17 people.

Meanwhile, Oil Minister Ibrahim Bahr al-Uloum met in Najaf with top Shiite cleric Grand Ayatollah Ali al-Sistani (search) and said the government will build a refinery in the holy city.