WASHINGTON - Afghan President Hamid Karzai must take significant steps to fight corruption, the U.S. military's top officer said on Wednesday, suggesting Washington was concerned inaction could undercut the campaign against the Taliban.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said it was too early to tell what Karzai and other leaders have done to address the issue, underscoring persistent tensions between Washington and the Kabul-based government on the issue.

There have to be significant steps taken on the part of President Karzai and other leaders in Afghanistan to eliminate corruption, Mullen told an audience at Kansas State University in an address broadcast to journalists at the Pentagon.

It's not going to go away overnight, he said of the corruption problem. But where it has not been addressed before, it is being addressed now, and we can't move forward in a positive way unless it continues to be.

He was duly elected by his people and he now has to perform in this area, Mullen said of Karzai, who has faced his own credibility challenges after voter fraud marred his re-election last year.

The Obama administration has been careful not to snipe publicly at Karzai, wanting to show U.S. backing for the new government, particularly as U.S. and NATO forces began a new campaign to push Taliban fighters out of population centres in the South.

The military aspect of this cannot succeed without success in other areas, Mullen said.

In addition to the corruption issue, the United States has been critical of several recent moves by Karzai's government, including a decision to block foreign observers from a U.N.-backed election watchdog group and planned curbs on media freedom.

(Reporting by Adam Entous; Editing by Cynthia Osterman)