President Barack Obama has said US tax collection agency's targeting of conservative groups for extra scrutiny before an election was "outrageous".
At the White House, Mr Obama said the Internal Revenue Service (IRS) needed to operate with "absolute integrity".
Groups with the words "Tea Party" or "patriot" in their names were singled out, the tax agency has acknowledged.
He also said criticisms of the response to an attack on a US mission in Libya last year were politically motivated.
Official talking points about the 11 September assault against the diplomatic facility in Benghazi, in which four Americans died, were edited by the state department to remove references to terrorism, it has been revealed.
Republicans have alleged the Obama administration sought to downplay terrorism ahead of the November presidential election.
'No patience'
At a White House news conference on Monday alongside UK Prime Minister David Cameron, Mr Obama said the IRS had to be held "fully accountable" if it had singled out tax-exempt groups for political reasons during the 2012 election campaign.
He said US taxpayers had to have confidence the tax agency was applying the law in a non-partisan fashion.
You don't want the IRS ever being perceived to be biased and anything less than neutral, Mr Obama said. "This is something that I think people are properly concerned about.
I've got no patience with it, I will not tolerate it and I will make sure that we find out exactly what happened.
The number of groups filing with the IRS for tax-exempt status shot up between 2010-12, ahead of last year's election.
The Tea Party is a loosely organised movement of anti-tax conservative activists, and the word "patriot" in US politics has largely come to signify a conservative stance.
It has been revealed the agency was checking to see if the conservative political groups were violating their tax-exempt status.
The head of the IRS tax-exempt office has said the practice was started in a branch office in Cincinnati, Ohio, where most of the applications were processed, and was not motivated by political bias.
The Associated Press news agency has reported senior IRS officials knew the agency was targeting Tea Party groups as early as 2011. A report from the US Treasury Department's inspector general is expected this week.
Republicans have promised to investigate, including a hearing scheduled by the House Ways and Means Committee for Friday.
Among those expected to testify is Steven Miller, the acting head of the IRS.
He wrote several letters to Congress last year over questions about the agency's procedure for handling tax-exemption applications.
But he never mentioned that conservative groups were being specifically targeted, a fact that was known to people he oversaw, according to the inspector general's report, obtained by the Associated Press.
We are going to need to find out how much he knew, said Representative Dave Camp, the chair of the committee.
Republican National committee chairman Reince Priebus said on Monday the Obama administration "continues to lose credibility by failing to answer even the simplest questions".
'Political circus'
On Benghazi, Mr Obama said that Republican concerns over talking points prepared for members of Congress and the news media were a "side show" and a "political circus".
The controversy stems in large part from remarks by US Ambassador to the UN Susan Rice several days after the 11 September assault.
Ms Rice said on television the attack grew out of an anti-US protest, even as other officials have said they knew at the time it was perpetrated by an organised, armed militant gang.
On Monday, Mr Obama said the idea that his administration had engaged in a cover-up lasting only three days "defies logic".
He said his administration had disclosed the emails related to the talking points to a congressional committee months ago.
And suddenly, three days ago, this gets spun up as if there's something new to the story, Mr Obama said.
The fact that this keeps on getting churned out, frankly, has a lot to do with political motivations, he added.
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