The Trump administration's "intention is never to lie to you", White House press secretary Sean Spicer told journalists during his first daily press briefing.
Mr Spicer faced questions on Monday during the briefing after a statement in which he denounced the media's correct reporting that the crowd at Donald Trump's inauguration was smaller than at his predecessor's eight years ago.
Mr Spicer said "sometimes we may disagree" about facts and said he wanted to have a "healthy relationship" with the White House press corps.
He added that "if we make a mistake, we'll do our best to correct" it.
The press secretary said he was given incorrect information about the number of passengers using Washington Metro's when he addressed the issue on Saturday but insisted that, when TV and online viewers are combined, that it was the most-watched inaugural in history.
Mr Spicer, a long-standing Republican operative who most recently was the spokesman for the Republican National Committee and also worked for former president George W Bush, is known for fighting tenaciously for his employers. His briefing on Saturday followed an appearance by Mr Trump at the CIA where the president hit out at the media for reporting his criticisms of the intelligence community.
He also took exception to stories saying the crowd for Friday's inauguration was smaller than those for his Barack Obama. Mr Trump declared that journalists are "the most dishonest human beings on Earth," and added: "I have a running war with the media."
Mr Spicer made two unprovable statements in his briefing: that photographs of the audience at Mr Trump's inauguration were intentionally framed to minimise the appearance of support, and that the new president had drawn the largest audience ever to witness an inauguration.
But he also made three statements which were quickly refuted: that the Washington Metro system recorded more riders on the day of Mr Trump's swearing-in than when Mr Obama was inaugurated in for his second term, that Friday was the first time that white floor covering was used on the Washington Mall - amplifying empty spaces - and that it was the first time spectators were required to pass through magnetometers to enter the Mall.
Mr Spicer's briefing, during which he did not take questions from reporters, was televised live on Fox News Channel and MSNBC. CNN did not air the session but showed highlights later.
Mr Trump's first press conference after he was elected, held on January 11, also took aim at the media. Coming hours after news reports revealed intelligence officials had presented Mr Trump with unsubstantiated and salacious allegations regarding his relationship to Russia, Mr Trump and his team condemned news organisations which disclosed details, calling out CNN and BuzzFeed as "disgraceful" and refusing to take questions from a CNN reporter.
Confronted by Meet the Press host Chuck Todd on Sunday with "falsehoods" stated by Mr Spicer, Trump adviser Kellyanne Conway called them "alternative facts". She accused Mr Todd of laughing at her and said he symbolises how the US president has been treated by the media.
Former White House press secretary Ari Fleischer said it seemed clear to him that Mr Spicer was acting on orders from his boss.
Press secretaries have to walk a fine line between reflecting the thinking and wishes of the president while trying to help the people covering him do their jobs, said Mr Fleischer, who also worked for George W Bush.
Mr Fleischer said he never knowingly delivered false information to the press while at the White House.
"You can't do that," he said. "It will shorten your career."
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