Seeking to quell a growing scandal, President Barack Obama said Monday it would be "outrageous" if US tax authorities had targeted conservative groups opposed to his White House.
As he battled growing political woes, Obama also denied his team had engaged in a cover-up to downplay the impact to his re-election campaign of the attack on the US mission in Benghazi last year, which killed four Americans.
Obama sought to defuse Republican fury and political damage to his administration over revelations that the Internal Revenue Service had subjected conservative grassroots groups to extra scrutiny.
"If, in fact IRS personnel engaged in the kind of practices that have been reported on and were intentionally targeting conservative groups, then that's outrageous," Obama said after meeting British Prime Minister David Cameron.
"They have to be held fully accountable because the IRS, as an independent agency, requires absolute integrity and people have to have confidence that they're applying it in a nonpartisan way."
The president said he first learned of the scandal last Friday from news reports. His spokesman Jay Carney said that the White House found out about it in April, though the information was not passed on to his boss.
Republicans, spotting a chance to damage Obama, have seized on drama, seeing an opportunity to fire up grassroots support.
Senator Marco Rubio, a possible 2016 presidential candidate, demanded the resignation of acting IRS commissioner Steven Miller.
"It is clear the IRS cannot operate with even a shred of the American people's confidence under the current leadership," Rubio said in a letter to Treasury Secretary Jack Lew.
"The American people deserve answers about how such seemingly unconstitutional and potentially criminal behavior could occur, and who else was aware of it throughout the administration."
The top Senate Republican, Mitch McConnell, told National Review that officials took their "cues from the tone expressed by the president, and he's made it clear that this administration is perfectly willing to crack down on critics."
The IRS, an independent agency within the US Treasury, admitted on Friday that it targeted around 75 groups associated with the ultra-conservative Tea Party movement, but denied it was driven by partisan motives.
Obama has also been hit by claims that his administration tried to downplay the political impact of the Benghazi raid, and changed talking points given to a top official to cast doubt on whether the attack was terrorism.
The president said that the campaign by Republicans and conservative media, which hit new heights with House of Representatives hearings on the issue last week, was politically motivated.
"The whole issue of talking points frankly throughout this process has been a sideshow," Obama said.
The White House had previously claimed that the talking points on the affair given to UN ambassador Susan Rice before she went on talk shows in September were only changed for stylistic reasons.
Republicans say her statement that the attack appeared to be spontaneous and a reaction to an anti-Muslim YouTube video deliberately ignored evidence that it was the work of Islamic extremists.
The attack in Libya came at a crucial time of Obama's re-election campaign when he was running on a platform of keeping Americans safe and largely neutralizing the threat from Al-Qaeda.
Obama said that suggestions the White House was engaged in a cover-up did not make sense because he said he sent a top aide to Capitol Hill three days after the attack to detail evidence that it was plotted by extremists.
"The whole thing defies logic and the fact this keeps on getting churned out frankly has a lot to do with political motivations," he said.
Several senior US officials testified last week that Washington could have done more to safeguard its mission in Benghazi ahead of and during the attack on September 11, 2012 which killed US ambassador Chris Stevens.
The White House argues that the circumstances of the attack have been probed by a review board ordered by then-secretary of state Hillary Clinton, and its recommendations had been acted upon.