"Afghanistan is much more complex," said Navy Adm. Mike Mullen, chairman of the Joint Chiefs of Staff.

He added: "I certainly recognize _ and having been in Vietnam myself _ that there are those who make comparisons. I would be pretty careful about that though, for lots of reasons."

The Pentagon is preparing to deploy an additional 15,000 Army and Marine troops to Afghanistan this spring and summer in the Obama administration's military campaign to shut down the Taliban and al-Qaida.

Ultimately, an estimated 60,000 U.S. troops could be in Afghanistan over the next year as Obama starts ordering soldiers from Iraq. About 32,000 American troops already are in Afghanistan.

Speaking to a Washington meeting of the Reserve Officers Association, Mullen stopped short of predicting how long U.S. troops would stay in Afghanistan. He said the main difference between Afghanistan and Vietnam is that "we are not an occupying force."

"We have no intention of that," Mullen said. "There isn't any of the 42-plus countries who are there that have that intention. ... That said, we cannot send a message to the Afghan people that we are."

Chief among the concerns, Mullen said, is making sure Afghanistan never again becomes a haven for al-Qaida leaders who moved to lawless Pakistan tribal regions in the post-9/11 hunt for Osama bin Laden.

"We cannot accept that al-Qaida leadership which continues to plan against us every single day _ and I mean us, here in America _ to have that safe haven in Pakistan nor could resume one in Afghanistan," Mullen said.

Efforts to eliminate government corruption and develop the poor nation also marks a contrast between the U.S. mission in Afghanistan from Vietnam, Mullen said.

Defense Secretary Robert Gates met Monday with President Barack Obama, but White House spokesman Robert Gibbs would not say whether the two discussed troop levels in Afghanistan.

Meanwhile, the Pentagon released a long-awaited study Monday describing crumbling security and a peak in violence in Afghanistan in spring and summer of 2008.

The quarterly status report, required by Congress, focused mostly on data available between April and September 2008 but included some year-end details, including:

_Between January and December 10, 2008, 132 U.S. personnel in Afghanistan died as the result of hostile action, up from 82 in 2007.

_The Afghan National Army Air Corps is beefing up its reconnaissance and gunship fleets and added 27 new helicopters and cargo planes by the end of December.

_As of December, NATO had provided only 42 Operational Mentor Liaison Teams out of 103 promised to train the Afghan National Security Forces.

_The annual Failed State Index, published by the Fund for Peace, showed worsening governance in Afghanistan between 2006 and 2008.

Mullen also said the global financial crisis is threatening U.S. security options abroad, forcing a delicate balance between national security and federal budget cuts.

"I am extremely concerned that in fact one of the outputs of this financial crisis will be an increase in instability," Mullen said.

He added: "Clearly, in listening to our new president, it's going to have an impact on the budgets in our government."