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 risen 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 U.S. soldiers and a journalist were killed when a huge bomb exploded beneath their Stryker on May 6.

"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.

The Army is forming seven Stryker brigades, each with more than 300 vehicles and 3,000 soldiers. One of them is the 56th Stryker Brigade, composed of Pennsylvania National Guard troops at armories across much of the state.

Capt. Cory Angel, a state Guard spokesman, said yesterday that the brigade would be ready for deployment, if required, by the end of next year.

But the Army and the Marines are already looking for something different that can survive big roadside bombs - the main threat to troops 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, which carry two crew members and 11 infantrymen, offer mobility, firepower and comfort.

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

"I love Strykers," said Spec. Christopher Hagen, based in Baqubah. "With Strykers, you're mobile, you're fast. You can get anywhere, anytime. They bring a lot of troops to the fight."

Some analysts have long questioned the wisdom of moving away from more heavily armored tracked vehicles, such as tanks and Bradley fighting vehicles, to wheeled transports such as 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.

"The Stryker vehicle was conceived at a time when the Army was more concerned about mobility and agility than it was about protection," said Loren Thompson, a military analyst from the Lexington Institute. "Stryker was the answer to that need."

The Stryker's vulnerabilities have become increasingly apparent since a battalion of about 700 soldiers and nearly 100 Stryker vehicles from the Army's Second Infantry Division was sent to Diyala province in March to bolster an infantry brigade struggling to restore order there. One day of engagement ended with one U.S. soldier dead, 12 wounded, and two Strykers destroyed.

Lt. Col. Bruce Antonio, who commands a Stryker battalion in Diyala, said he and his troops still had confidence in the Strykers and noted they had survived many bombs.

But Antonio said some insurgents had found "the right mix of explosives and . . . positioning to inflict severe damage on the vehicle." He also noted that tanks had also proved vulnerable.

The insurgents also apparently are becoming better at hiding the devices - the bomb that killed the six soldiers and the journalist was believed hidden in a sewer line. To add potency, insurgents surrounded the device with concrete to channel the blast force up into the vehicle, according to soldiers familiar with the investigation.

Supporters of the Strykers say all that proves that it is the lethality of bombs in Iraq - not the Strykers themselves - that is the problem.

The Army and Marine Corps already are pushing for new Mine Resistant Ambush Protected vehicles, or MRAPS, whose V-shaped hulls are designed to deflect bomb blasts outward, rather than through the vehicle.

The Pentagon has requested nearly 7,800 of the new vehicles at a cost of $8.4 billion and is considering ordering thousands more to give troops better protection.

To compare Strykers with other military vehicles, go to http://go.philly.

com/strykerEndText

Inquirer staff writer Tom Infield contributed to this article.