The head of the US tax agency has quit after it emerged his staff singled out conservative groups for extra scrutiny, President Barack Obama has announced.Treasury Secretary Jack Lew had asked for and accepted the resignation of Internal Revenue Service (IRS) Acting Commissioner Steve Miller, he said, BBC informs. “I will do everything in my power to make sure nothing like this happens again,” Obama told a news conference.The scandal has been one of several to rattle the White House in recent days. Earlier on Wednesday, US Attorney General Eric Holder faced four hours of questioning at a Congressional hearing on the IRS, the secret seizure of phone records from the Associated Press news agency, and the attack on the US consulate in the Libyan city of Benghazi.Mr Holder told the House judiciary committee that it would take time for the FBI to determine if any laws had been broken by IRS personnel. In a short statement to reporters at the White House, President Obama said he had reviewed the Treasury Inspector General for Tax Administration (TIGTA) report on the IRS’s targeting of conservative groups and found the “misconduct” uncovered was “inexcusable”.“Americans are right to be angry about it, and I am angry about it,” he said. “I will not tolerate this kind of behaviour in any agency, but especially in the IRS, given the power that it has and the reach that it has into all of our lives.” “The IRS has to operate with absolute integrity.”To that end, Mr Obama revealed that the treasury secretary had requested and accepted the resignation of acting commissioner of the IRS.