At least 7 copters hit by militants' fire since Jan. 20

Militants take aim on helicopters from several directions.
BAGHDAD, Iraq (AP) -- A U.S. helicopter was shot down and crashed north of Baghdad on Wednesday, the military said after initially stating that the chopper made a "hard landing." All aboard were safely evacuated by a second helicopter.
At least seven U.S. helicopters have crashed or been forced down under hostile fire since Jan. 20. Military officials have said that militants are increasingly targeting helicopters amid the buildup of U.S. troops in Baghdad, firing simultaneously with an assortment of weapons from various directions.
The military has also detected another deadly insurgent tactic in recent weeks -- the spreading of toxic chlorine gas by combining it with explosives.
In Washington, two Pentagon officials said the tactic has been used at least three times since Jan. 28, when a truck carrying explosives and a chlorine tank blew up in Anbar province west of Baghdad. More than a dozen people were reported killed.
On Tuesday, a tanker filled with chlorine exploded and noxious plumes covered homes and schools north of Baghdad. Nine people were killed and 150 people were wounded, said one of the officials. A day later, a pickup truck exploded near a diesel fuel station in southwestern Baghdad, the official said.
A third defense official said the U.S. has been concerned about militants' ability to acquire weapons such aschlorine bombs. But so far, the official said, bomb makers in Iraq haven't been able to disperse a chemical such as chlorine in an effective way.
All three officials spoke on condition of anonymity because of the sensitivity of the information.
Pullouts announced
Meanwhile, Britain outlined its plan to withdraw around 1,600 troops from Iraq in the coming months, and Denmark said it will withdraw its 460-member contingent by August. Lithuania also said it may pull back its 53 troops from the country.
Secretary of State Condoleezza Rice said that despite the announced withdrawals, "the coalition remains intact." In Japan, Vice President Dick Cheney said the U.S. wants to finish its mission in Iraq, then "come home with honor."
Political tremors also grew stronger in Iraq following claims that a Sunni woman was raped while in custody of the Shiite-dominated police -- a case that threatens to escalate the sectarian friction that drives many of the bombings and attacks across the country.
Prime Minister Nouri al-Maliki fired the head of the influential Sunni Endowment, who had called for an international investigation into the rape allegations.
More violence
In the latest violence, a suicide car bomber struck a police checkpoint in the Shiite holy city of Najaf, killing 13 people in the spiritual heartland of the militia factions led by radical cleric Muqtada al-Sadr.
The blast hit while streets were filled with morning shoppers. At least seven of the victims were police and the rest civilians, authorities said. It was the first large-scale bombing in months in the city, which is heavily guarded by police and al-Sadr's powerful Mahdi Army militia. More than 40 people were injured.
Government officials marked the first week of a wide-ranging security sweep in Baghdad by U.S. and Iraqi forces seeking to put death squads and insurgents on the run. But a string of bombings in the Baghdad area -- which have claimed more than 100 lives since Sunday -- have quieted the early fanfare and highlighted the huge challenges of trying to reclaim control of the blood-soaked capital.
A car bomb in the western Baghdad district of Bayya killed at least two and injured 31, police said. Later, a car bomb in the neighborhood killed at least three people.
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