WASHINGTON (Reuters) - U.S. assertions about the growing independence of Iraqi security units are often based on conflicting claims or assessment methods that do not allow for “independent” ratings, a U.S. watchdog agency said on Monday.

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A report by the Government Accountability Office said the Defense Department and top military officials including U.S. Commander in Iraq Gen. David Petraeus have sought to show progress by repeatedly citing numbers of Iraqi Security Force (ISF) units deemed “independent” or “fully independent.”

But it is unclear how the Pentagon and its officials have reached those conclusions, according to the GAO.

“First, the process by which ISF units are assessed does not allow for a rating of ‘independent’ or ‘fully independent.’ Second, statements in DOD reports seemingly contradict claims of ISF independence,” said the agency, a nonpartisan investigative arm of Congress.

The GAO specifically pointed to Petraeus’ September testimony to Congress in which he told lawmakers that coalition forces have assessed one ISF unit to be “fully independent” each month since November 2005 with only one exception.

“Without clarity regarding the criteria according to which ISF units are assessed as independent ... Congress cannot have clear visibility over DOD’s role in assisting the ISF in becoming independent of coalition support,” the 33-page report said.

The United States has spent $19.2 billion since 2003 to train and equip about 350,000 Iraqi soldiers and police in hopes of deploying an effective security force that would ultimately allow for the withdrawal of U.S. troops from Iraq.

But with the Bush administration now beginning to withdraw 20,000 extra combat troops sent to Iraq to quell sectarian violence, the GAO report said there is no reliable data on how many of those Iraqi security forces are still serving.

The greatest obstacle preventing Iraqi security units from achieving full independence has been their lack of logistical support, intelligence and command and control structures.

The GAO said there has been some progress in these areas but that the Iraqi government has been stymied by continued sectarian violence that has led to a larger force at the expense of developing support capabilities.

The Ministry of Defense, which oversees the Iraqi Army, has diverted recruits intended for logistics into combat, understaffed training schools, experienced a growing shortage of noncommissioned officers.

Meanwhile, violence has prevented the Ministry of Interior, which controls the Iraqi police, from completing a command and control network and developing its intelligence capabilities.