Baghdad ? A string of heavy losses from powerful roadside bombs has raised new questions about the vulnerability of the Stryker, the Army’s troop-carrying vehicle hailed by supporters as the key to a leaner, more mobile force.

Since the Strykers went into action in violent Diyala province north of Baghdad two months ago, losses of the vehicles have been rising steadily, U.S. officials said.

A single infantry company in Diyala lost five Strykers this month in less than a week, according to soldiers familiar with the losses, who spoke on condition of anonymity because they are not authorized to release the information. The overall number of Strykers lost recently is classified.

In one of the biggest hits, six American soldiers and a journalist were killed when a huge bomb exploded beneath their Stryker on May 6. It was the biggest one-day loss for the battalion in more than two years.

“We went for several months with no losses and were very proud of that,” a senior Army official said in Washington, speaking on condition of anonymity because he is not authorized to comment publicly. “Since then, there have been quite a few Stryker losses.”

“They are learning how to defeat them,” the Army official said of Iraqi insurgents.

The military introduced the eight-wheeled Stryker in 1999 as the cornerstone of a ground force of the future – hoping to create faster, more agile armored units than tank-equipped units, but with more firepower and protection than light-infantry units. The Army has ordered nearly 2,900 vehicles for its

$13 billion Stryker program.

But the Army and the Marines are already looking for something different that can survive big roadside bombs – the main threat to soldiers in Iraq – meaning the Stryker’s high-profile status as the Army’s “next generation” vehicle may be short-lived.

“It is indeed an open question if the Stryker is right for this type of warfare,” said Michael O’Hanlon, a senior analyst with the Brookings Institution. “I am inclined to think that the concept works better for peacekeeping. But based on data the Army has made available to date, it’s hard to be sure.”

Supporters of the Strykers, which have been used in Iraq since late 2003, say the vehicles that carry two crew members and 11 infantrymen offer mobility, firepower and comfort.

Lighter and faster than tracked vehicles like tanks, each Stryker can rush soldiers quickly to a fight, enabling commanders to maintain security over a wide area with relatively fewer troops. Humvees can carry only four soldiers – and are more vulnerable to bombs even when their armor is upgraded.

“I love Strykers,” said Spc. Christopher Hagen, based in Baqouba. “With Strykers, you’re mobile, you’re fast. You can get anywhere anytime. They bring a lot of troops to the fight.”

But some analysts have long questioned the wisdom of moving away from more heavily armored tracked vehicles like tanks and Bradley fighting vehicles to wheeled transports, like the Stryker.

They say that is especially true in Iraq, where powerful bombs – not rocket-propelled grenades or small arms fire – are the main threat.