The U.S. office overseeing billions of dollars for Afghanistan's rebuilding lacks leadership and focus at a time when aggressive, independent oversight of the country's reconstruction is more important than ever, three senators told President Barack Obama on Tuesday.

The office of the special inspector general for Afghanistan reconstruction, headed by retired Marine Corps Gen. Arnold Fields, has failed to hire enough staff, has issued too few audit and investigative reports, and may be losing sight of its mission, they say in a Dec. 8 letter to Obama.

In a statement, Fields said the letter from Sens. Claire McCaskill, D-Mo., Susan Collins, R-Maine, and Tom Coburn, R-Okla., paints an inaccurate picture. The office "started from scratch with minimal funding," Fields said, adding that he has formed an experienced team that is helping to improve the reconstruction effort.

The inspector general's office is responsible for monitoring a broad range of projects, including training of the Afghan army and police, and ensuring U.S. tax dollars are spent properly. The office was created by Congress in 2008, nearly seven years after U.S. forces invaded Afghanistan.

In their letter to Obama, obtained by The Associated Press, the senators don't call for Fields' resignation. But they do want the White House to conduct a thorough review of the office "to determine if improvements can be made to the organization."

The White House did not immediately respond to a request for comment.

The senators are members of the Senate Homeland Security and Governmental Affairs Committee.

The criticism of Fields' office comes as the Obama administration is escalating the U.S. mission in Afghanistan to stabilize the government and defeat the Taliban insurgency. But corruption with Afghan President Hamid Karzai's government and the primitive state of much of the country's infrastructure have led to concerns the goals may not be met even with more money and people.

According to the most recent quarterly report to Congress from Fields' office, the U.S. has committed $39 billion for reconstruction programs in Afghanistan. That figure is expected to hit $50 billion in 2010.

Without vigorous oversight by an experienced staff, the senators say, the rebuilding of Afghanistan will run into the same problems that occurred in Iraq, where nearly $50 billion was spent on reconstruction projects marred by waste and fraud.