U.S. Defense Secretary Donald Rumsfeld is appearing before Congress to testify about the abuse of Iraqi prisoners in U.S. custody.

Mr. Rumsfeld is expected to face tough questions during his appearances Friday, before the Senate and House Armed Services committees.

Officials say Mr. Rumsfeld will apologize to Congress for not keeping it informed about the abuse, and call for creation of an independent commission to review how the Defense Department handled the abuse cases.

In an opening statement, committee member Senator John Warner said the misconduct of the soldiers who carried out the abuse is as serious as any he has ever seen.

Controversy over the mistreatment has sparked calls for Mr. Rumsfeld's resignation, and compelled President Bush to issue a public apology.

Lawmakers say Mr. Rumsfeld's testimony will be crucial to his survival as defense secretary. The committees are expected to press Mr. Rumsfeld about the extent of prisoner abuse at Baghdad's Abu Ghraib prison, steps taken to correct the problem, and why lawmakers were not informed about the mistreatment earlier.

The controversy began last week after U.S. television network aired photos of Iraqi prisoners, some naked, posed in humiliating positions.

In addition to Mr. Rumsfeld, the committees will hear Friday from General Richard Myers, chairman of the U.S. military's Joint Chiefs of Staff.

Among those calling for Mr. Rumsfeld to resign are the top Democrat in the House of Representatives, Nancy Pelosi, and a prominent U.S. Muslim group, The Council on American-Islamic Relations.

President Bush has rejected calls for Mr. Rumsfeld's resignation, saying he is a valuable member of the cabinet.

Mr. Bush Thursday issued a rare public apology for the Iraqi prisoner abuse. Speaking at the White House after talks with Jordan's King Abdullah, Mr. Bush said he was "sorry" for the incidents, called them a stain on the country's honor and vowed to punish those responsible.