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U.S. military officials fear that the closure of inner-city bases and restrictive guidelines that go into effect this week will leave American troops and civilians in Iraq more vulnerable.

Of particular concern is a new rule that bars U.S. troops from using mine-resistant armored vehicles in urban areas during the day, officials said.

Also worrisome, they said, is the recent closure of a small outpost in eastern Baghdad that is adjacent to a site that militiamen have used to launch deadly rocket attacks on the Green Zone.

Thousands of U.S. combat troops will remain at a handful of bases in Baghdad and on the outskirts of other restive cities, such as Mosul and Kirkuk, in northern Iraq, past the June 30 deadline. But U.S. troops say their ability to respond quickly to thwart attacks could erode significantly because Iraqi officials will have unprecedented authority over their mobility and missions in urban areas.

“We won’t be providing the same level of security for ourselves and Iraqis,” said 2nd Lt. Jason Henke, a military police platoon leader who will remain at one of the few inner-city bases in Baghdad. “With only a small window of time that we are allowed to operate in, it’s going to be easier to target U.S. forces when we are outside the wire.”

Henke’s concerns were heightened this week by a string of powerful roadside bombings near his base in central Baghdad.

On Tuesday, one of his squadrons was attacked with an armor-piercing bomb that struck the passenger side of a mine-resistant armored vehicle, igniting the fuel line. His platoon lost another truck Thursday in a similar attack.

As other soldiers rushed to help their comrades out of the burning vehicle, insurgents opened fire with AK-47 assault rifles.

The soldiers returned fire and escaped without injuries, said Henke, 29, of Newbury Park, Calif. But had they been in a Humvee, a smaller, less-fortified vehicle that soldiers will use during daytime as of next month, “they would have all been killed,” Henke said.

Brig. Gen. Stephen Lanza, the top U.S. military spokesman in Iraq, said U.S. troops would depend on their Iraqi counterparts more than ever to detect and respond to threats.