WASHINGTON (Reuters) - U.S. senators questioned State Department officials closely on Thursday about corruption in Afghanistan and said failure to address it could cause them to rethink the billions of dollars the United States spends there each year.

"I don't know what the political will here in the United States will be to continue to support the Afghans in light of what is going on there," said Democratic Senator Robert Menendez, a senior member of the Senate Foreign Relations Committee.

Menendez described himself as someone who has been supportive of U.S. Afghanistan policy, but said he would "have a totally different view" if the government in Kabul does not act.

On Wednesday, the Special Inspector General for Afghanistan Reconstruction (SIGAR) issued a report that strongly criticized Washington for pouring billions of dollars into Afghanistan with so little oversight that it fueled corruption and undermined the U.S. mission.

The United States spends $5 billion per year in Afghanistan, about $4 billion for defense and national security and another $1 billion in civilian assistance, plus billions more for the cost of the thousands of troops and military contractors there.

Richard Olson, the State Department's special representative for Afghanistan and Pakistan, agreed that corruption can undermine governance. But he called Afghan President Ashraf Ghani "a committed partner" in fighting corruption.

Ghani, Olson said, has taken steps including addressing the Kabul Bank scandal, canceling a fuel contract and setting up a monitoring and evaluation committee with experts on anti-corruption.

"It is a dramatically different situation from what it was prior to 2014," Olson testified.

(Reporting by Patricia Zengerle; Editing by Bill Rigby)