Michael Gilmore, the Pentagon's top weapons tester, assessed the LPD-17 amphibious warfare ship as being "not effective, not suitable, and not survivable in a combat situation," a Defense Department spokeswoman said.
The reasons for the assessment are classified, spokeswoman Cheryl Irwin said in an email.
The ship, designed to carry Marines and landing craft, was found to be capable of conducting amphibious operations in a "benign environment," she said.
The 10 planned ships of the LPD-17 San Antonio class are a key element of the US Navy's ability to project power, including for humanitarian assistance and disaster response, Reuters reports.
The Marines have not conducted a large-scale amphibious assault since the 1950-53 Korean War.
Defence Secretary Robert Gates has repeatedly questioned the future of such forcible U.S. landings, "especially as advances in anti-ship systems keep pushing the potential launch point further from shore," as he put it in May.
The LPD-17's shortcomings were described in a "Combined Operational and Live Fire Test and Evaluation" report sent to Congress in June.
It found they would stand up better under enemy fire than its four classes of predecessor ships but ultimately lacked survivability, Irwin said.
Northrop and General Dynamics Corp are the military's main shipbuilders. The LPD-17 program long has been troubled by cost growth, schedule slips and construction problems, particularly on the earlier ships in the program.
Northrop Grumman, the Pentagon's No. 3 supplier by sales, referred questions to the Navy.
The Navy has reviewed the report and "is taking the necessary steps to implement required actions across the LPD 17 class to improve the ship's survivability in a combat situation," said Navy spokeswoman Lieutenant Courtney Hillson.
Northrop shares closed 1.56 percent higher at $63.21 on the New York Stock Exchange.
Northrop Chief Executive Wes Bush said in July the company was exploring a possible spinoff or sale of its shipbuilding business. He said it would close its yard in Avondale, Louisiana, by 2013 to cut excess capacity, and consolidate its Gulf Coast shipbuilding operations in Mississippi.
At least four possible private-equity buyers have emerged, including Carlyle, KKR, TPG and Bain Capital as well as Cleveland Ship LLC, a small Ohio-based shipbuilder.
The first three ships of the class - San Antonio, New Orleans and Mesa Verde - have successfully completed overseas deployments, the Navy said.
Gilmore tested LPD-17 and LPD-19, Irwin, the Pentagon spokeswoman, said.
She said tests on later ships in the class may bring different results "because the Navy continues to work to correct problems."
The Navy's current five-year shipbuilding plan calls for buying an 11th and final LPD-17 in fiscal 2012, which starts Oct. 1, 2011, but Congress has not approved this.
The Navy estimates the price tag of the 11th ship at just over US$2 billion, according to an Oct. 4 report by the nonpartisan Congressional Research Service.
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