BAGHDAD (AP) - Three suicide truck bombers hit enclaves of a small religious sect Tuesday, killing at least 20 people as extremists across Iraq staged other bold attacks: leveling a key bridge and abducting five officials from an Oil Ministry compound in a raid using gunmen dressed as security forces.

The U.S. military also tried to press its gains against guerrillas. A vanguard of 16,000 U.S. and Iraqi soldiers began a sweep through the Diyala River valley north of Baghdad in pursuit of Sunni insurgents and Shiite militia fighters driven out of strongholds in the several weeks.

In Iraq's western desert, meanwhile, five soldiers were killed when a CH-47 Chinook helicopter went down Tuesday during a test flight. The cause of the crash was under investigation, said 1st Lt. Shawn Mercer, a Marine spokesman. A total of nine U.S. deaths were announced.

U.S. officials believe extremists are attempting to regroup across northern Iraq after being driven from strongholds in and around Baghdad. Such a retrenching could increase pressure on small communities such as the Yazidi sect, a primarily Kurdish group with ancient roots that worships an angel figure considered to be the devil by some Muslims and Christians.

The triple suicide attacks on the Yazidi community came just after sundown near Qahataniya, 75 miles west of Mosul, Iraq's third-largest city, said police Capt. Ghanim Riyadh and a top local official Abdul-Rahman al-Shimri.

At least one of the trucks was an explosives-laden fuel tanker, police said. Several shops were set ablaze and apartment buildings were crumbled by the powerful explosions. At least 70 people were reported injured.

There was no claim of responsibility, but the attacks bore the hallmark of al-Qaida in Iraq, which has been regrouping in the north after being driven from safe havens in Anbar and Diyala provinces.

The sect has been the targets of attacks in the past, with the most violent coming after the stoning death in April of a Yazidi teenager who had recently converted to Islam after she eloped with a Muslim. Police said the 18-year-old woman was killed by relatives who disapproved of the match.

Two weeks later, gunmen shot and killed 23 Yazidis execution-style after stopping their bus and separating out followers of other faiths in what was believed to have been retaliation for the woman's death.

The bodies of two Yazidi men who had been stoned to death also turned up in the morgue in the northern city of Kirkuk on Tuesday, six days after they had been kidnapped while they were en route to Baghdad to sell olives, police said.

"The two men killed were only peasants who were planning to sell their crops in Baghdad, they have nothing to do with political and religious disputes. We are still paying the price of a foolish, wrong act conducted by small number of Yazidis who stoned the woman," said 44-year-old Sami Benda, a relative of one of the slain men.

The center of the Yazidi faith is around Mosul, but smaller communities exist in Turkey, Syria and other places.

Baghdad was spared major violence in another sign that a six-month-old security crackdown in the capital was disrupting extremists' firepower. But the brazen daylight raid on the Oil Ministry complex showed that armed gangs can still embarrass authorities.

Dozens of gunmen wearing security forces uniforms stormed an Oil Ministry compound Tuesday and abducted a deputy oil minister and four other officials who were spirited away in a convoy of military-style vehicles.

The kidnappings - similar to a commando-like raid into Iraq's Finance Ministry in May - included Abdel-Jabar al-Wagaa, a senior assistant to Oil Minister Hussain al-Shahristani, said Assem Jihad, the oil ministry spokesman.

Al-Wagaa and four other officials with the State Oil Marketing Organization were taken away by more than 50 gunmen in military-style vehicles, said an Interior Minister official, speaking anonymously because he was not authorized to release the information. Five bodyguards were wounded in the raid, the official said.

On May 29, five Britons were seized May 29 in a similar raid on Iraq's Finance Ministry. They were taken by gunmen wearing police uniforms and have not been found.

Both government organizations are near the Sadr City Shiite enclave, a stronghold of the Mahdi Army militia.

The raid also was reminiscent of an attack by Mahdi Army fighters, dressed as Interior Ministry commandos, who stormed a Higher Education Ministry office Nov. 14 and snatched away as many as 200 people. Dozens of those kidnap victims were never been found.

Just north of the capital, a suicide truck bomber blew apart a strategic bridge on the highway linking Baghdad with Mosul. Police said at least 10 people died. The Thiraa Dijla bridge in Taji - near a U.S. air base 12 miles north of the capital - was bombed three months ago, leaving only one lane open.

In western Iraq, a U.S. transport helicopter crashed near an air base, killing five troopers, the military said. The CH-47 Chinook helicopter was conducting a routine post-maintenance test flight when it went down near Taqaddum air base, the U.S. military said.

Four U.S. soldiers were reported killed in separate attacks - three in an explosion near their vehicle Monday in the northwestern Ninevah province. The fourth died of wounds sustained in combat in western Baghdad.

The deaths raised to at least 3,700 members of the U.S. military who have died since the Iraq war started in March 2003, according to an Associated Press count.

The violence punctuated a day when 16,000 U.S. and Iraqi troops began a sweep through the Diyala River valley in a new operation north of Baghdad in pursuit of Sunni insurgents and Shiite militia fighters driven out of Baqouba and Anbar province over the past several weeks.

Lt. Col. Michael Donnelly, a military spokesman in northern Iraq, said the force included 10,000 U.S. troops and 6,000 Iraqis. He said U.S. aircraft used more than 30,000 pounds of ammunition to block routes and destroy known and suspected heavy machine gun positions.

The Air Force also dropped 9,000 pounds of bombs to attack what the military called an al-Qaida in Iraq training camp, which included bunkers, living quarters, weapons and ammunition caches, Donnelly said.

Three suspected militants had been killed and four booby-trapped houses destroyed, he said, citing preliminary results.

In Washington, Defense Department spokesman Bryan Whitman said the new operation was one in a series planned over the next 30 days to try to blunt expected attempts by al-Qaida in Iraq to influence events during "this critical period" as the top U.S. commander in Iraq, Gen. David Petraeus, plans his assessment for Congress.

"We fully expect that al-Qaida in Iraq would like to increase their attacks during this critical period," Whitman said Tuesday. "And this increased intensity in offensive operations … will take the fight to the enemy with the purpose of improving the overall security situation in Baghdad" as well as increase "pressure on al-Qaida in Iraq countrywide and prevent the enemy from conducting their own operations."

Associated Press writers Qassim Abdul-Zahra, Sameer N. Yacoub and Yahya Barzanji contributed to this report.