BAGHDAD — Four suicide bombers struck at communities of a small Kurdish sect in northwestern Iraq with nearly simultaneous attacks today, killing at least 175 people and wounding 200 more, Iraqi military and local officials said.

The death toll was the highest in a concerted attack since November 23, when 215 people were killed by mortar fire and five car bombs in Baghdad’s Shiite Muslim enclave of Sadr City. And it was most vicious attack yet against the Yazidis, an ancient religious community in the region whose members are considered infidels by some Muslims.

The bombings came as extremists staged other bold attacks: leveling a key bridge outside Baghdad and abducting five officials from an Oil Ministry compound in the capital in a raid using gunmen dressed as security officers.

The American military, meanwhile, sought to press its gains against guerrillas. Some 16,000 American 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 recent weeks.

American 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 Islamic State in Iraq, an Al Qaeda front group, distributed leaflets a week ago warning residents near the scene of today’s bombings that an attack was imminent because Yazidis are “anti-Islamic.”

The sect was under fire after some members stoned a Yazidi teenager to death in April. She had converted to Islam and fled her family with a Muslim boyfriend. Police said 18-year-old Duaa Khalil Aswad was killed by relatives who disapproved of the match.

A grainy video showing gruesome scenes of the woman’s killing was later distributed on Iraqi Web sites. Its authenticity could not be independently verified, but recent attacks on Yazidis have been blamed on Al Qaeda-linked Sunni insurgents seeking revenge.

The suicide attacks came just after sundown near Qahataniya, 75 miles west of Mosul, Iraq’s third-largest city, the top government official in the area, Abdul-Rahman al-Shimiri, and Iraq army Captain Mohammed Ahmed said.

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

Witnesses said American helicopters swooped in to evacuate wounded to hospitals in Dahuk, a Kurdish city near the Turkish border about 60 miles north of Qahataniya. Civilian cars and ambulances also rushed injured to hospitals in Dahuk, police said.

“I gave blood. I saw many maimed people with no legs or hands,” a 40-year-old Yazidi teacher who went to a hospital to donate blood, Ghassan Salim, said. “Many of the wounded were left in the hospital garage or in the streets because the hospital is small.”

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

“This is a terrorist act and the people targeted are poor Yazidis who have nothing to do with the armed conflict,” Dhakil Qassim, mayor of Sinjar, a town near the attacks, said.

He blamed Al Qaeda in Iraq, citing what he said were intelligence reports from the Kurdish regional government. “Al Qaeda fighters are very active in this area near the Syrian border.”

Two weeks after the Yazidi woman was stoned to death, gunmen 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 turned up in the morgue in the northern city of Kirkuk today, six days after they had been kidnapped while en route to Baghdad to sell olives, police said.

“We are still paying the price of a foolish, wrong act conducted by small number of Yazidis who stoned the woman,” a relative of one of the slain men, 44-year-old Sami Benda, said.

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 is 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 force uniforms stormed the compound 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 on Iraq’s Finance Ministry in May — included Abdel-Jabar al-Wagaa, a senior assistant to Oil Minister Hussain al-Shahristani, the oil ministry spokesman, Assem Jihad, said.

Mr. Al-Wagaa and four other officials with the State Oil Marketing Organization were taken away by more than 50 gunmen in military-style vehicles, an Interior Minister official said, 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 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 Sadr City, a stronghold of the Mahdi Army militia loyal to radical Shiite cleric Muqtada al-Sadr.

The raids were reminiscent of an attack by Mahdi Army fighters, dressed as Interior Ministry commandos, who stormed a Higher Education Ministry office November 14 and carried off as many as 200 people. Dozens of those kidnap victims were never been found.

Just north of the capital, a suicide truck bomber devastated a key bridge on the highway linking Baghdad with Mosul. Police said at least 10 people died. The Thiraa Dijla bridge in Taji — near an American air base 12 miles north of the capital — also was bombed three months ago, leaving only one lane open.

In western Iraq, an American 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 American military said.

Four other American soldiers were reported killed in combat — three in an explosion near their vehicle yesterday in the northwestern Ninevah province. The fourth died of wounds suffered in western Baghdad.

The deaths raised to at least 3,700 the number of American military personnel 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 American and Iraqi soldiers began a sweep through the Diyala River valley in a new operation north of Baghdad in pursuit of Sunni insurgents and Shiite militiamen driven out of Baqouba and Anbar province over the past several weeks.

Lieutenant Colonel. Michael Donnelly, a military spokesman in northern Iraq, said the force included 10,000 Americans and 6,000 Iraqis. He said American aircraft used more than 30,000 pounds of munitions to block routes and destroy known and suspected heavy machine gun positions.

The Air Force also dropped 9,000 pounds of bombs to attack an Al Qaeda 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 reports.

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 Qaeda in Iraq to influence events during “this critical period” as the top American commander in Iraq, General David Petraeus, plans his assessment for Congress.

“We fully expect that Al Qaeda in Iraq would like to increase their attacks during this critical period,” Mr. Whitman said today.

“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 Qaeda in Iraq countrywide and prevent the enemy from conducting their own operations.”