Story Highlights• NEW: UK: Two British military helicopters crashed Sunday, killing two
• At least 12 killed, 26 injured in attacks in northern cities of Baiji, Mosul
• Suicide bomber, bus bomb kill 17 in separate incidents in Baghdad
• Twin car bombs explode within minutes of each other in Baghdad, killing 18
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BAGHDAD, Iraq (CNN) -- A string of car bombs and suicide attacks across Iraq on Sunday killed and wounded dozens in what has been a bloody weekend for the war-ravaged nation.
Reports of the attacks came as the British Ministry of Defense announced that two British military helicopters apparently collided and crashed Sunday in a rural area on the northern outskirts of Baghdad, killing two British troops and injuring four others, one seriously.
Initial reports suggest the crash was an accident and enemy fire was not involved, British Defense Secretary Des Browne said.
The Puma choppers went down near Taji, a town 20 miles (32 kilometers) north of Baghdad, the U.S. military said.
The dead were from the Royal Air Force and British army, Browne said. The number of British troops killed in the Iraq war stands at 142.
Shoppers dazed by car bombs
Also Sunday morning, two car bombs exploded within minutes of each other in a shopping and dining area of southwest Baghdad, killing at least 18 people and wounding another 52 people, according to Baghdad police.
The first bomb hit a popular restaurant at about 10 a.m., while a second one exploded 10 minutes later and 100 yards away in an outdoor market in al-Shurta al-Rabeia district, police said.
Later, in northwestern Baghdad, a suicide bomber detonated an explosive vest on a small bus, killing six people and wounding 11, Baghdad police said. The bus was traveling between the predominantly Shiite neighborhoods of Autaifiya and Kadhimiya.
In central Baghdad's Karrada district, a minibus packed with explosives blew up on a commercial road, killing 11 people and wounding 15 others, a Baghdad police official said.
More U.S. soldiers dead
Also in the capital Sunday, small-arms fire killed an American soldier backing an Iraqi national police unit near a mosque, the U.S. military said. An Iraqi civilian also was wounded, the military said.
On Saturday, a U.S. soldier was killed when a roadside bomb exploded near troops conducting a foot patrol in southern Baghdad, the military said. Also Saturday a U.S. Marine died during combat operations in Anbar province, west of Baghdad, the military said.
Since the 2003 invasion of Iraq, 3,301 U.S. military personnel have died in the war, including seven Defense Department civilians.
In Baiji, north of Baghdad, four Iraqi soldiers were killed and five others were wounded Sunday when a suicide car bomber slammed into an army checkpoint, a Tikrit police official said.
The city, about 125 miles (200 kilometers) north of the capital, is in Salaheddin province and is home to Iraq's largest oil refinery.
In a separate attack, gunmen wounded the commander of Iraq's border police in Salaheddin province and killed four border police officers in an ambush on the commander's convoy Sunday afternoon.
In the northern city of Mosul, four people were killed and 16 others were wounded Sunday when two suicide car bombs exploded in quick succession at an Iraqi army base, police said. Two Iraqi soldiers were among the dead, a police official said.
Also in Mosul, four Iraqi soldiers were critically wounded when a car bomb detonated near their patrol, the official said.
Sunday's violence comes on the heels of a car bomb blast Saturday in a crowded shopping area of Karbala, killing at least 44 people and wounding 224, according to an official with the city's health directorate.
Ten women and 10 children were among those killed by the blast near a bus station and just 200 yards from the Imam Hussein shrine, where the grandson of the Prophet Mohammed is buried. Karbala is a holy Shiite city about 70 miles (113 kilometers) southwest of Baghdad.
A short time later, a car bomb exploded on the Jadriya bridge, which spans the Tigris River in southern Baghdad, killing at least 10 people and wounding 15 others, Iraqi police said.
That bridge attack came two days after a suicide car bomb detonated on the Sarafiya bridge, which crosses the Tigris in northern Baghdad, also killing 10 people. Two large sections of the bridge collapsed into the river.
Also Saturday, Baghdad police reported finding 14 bullet-riddled bodies around the Iraqi capital.
CNN's Mohammed Tawfeeq contributed to this report.