Russia's President Putin holds a glass during a ceremony of receiving diplomatic credentials from foreign ambassadors at the Kremlin in Moscow Thomson Reuters

WASHINGTON (Reuters) - The overseers of the U.S. intelligence community have not embraced a CIA assessment that Russian cyber attacks were aimed at helping Republican President-elect Donald Trump win the 2016 election, three American officials said on Monday.

While the Office of the Director of National Intelligence (ODNI) does not dispute the CIA's analysis of Russian hacking operations, it has not endorsed their assessment because of a lack of conclusive evidence that Moscow intended to boost Trump over Democratic opponent Hillary Clinton, said the officials, who declined to be named.

The position of the ODNI, which oversees the 17 agency-strong U.S. intelligence community, could give Trump fresh ammunition to dispute the CIA assessment, which he rejected as "ridiculous" in weekend remarks, and press his assertion that no evidence implicates Russia in the cyber attacks.

Trump's rejection of the CIA's judgment marks the latest in a string of disputes over Russia's international conduct that have erupted between the president-elect and the intelligence community he will soon command.

An ODNI spokesman declined to comment on the issue.

"ODNI is not arguing that the agency (CIA) is wrong, only that they can't prove intent," said one of the three U.S. officials. "Of course they can't, absent agents in on the decision-making in Moscow."

The Federal Bureau of Investigation, whose evidentiary standards require it to make cases that can stand up in court, declined to accept the CIA's analysis - a deductive assessment of the available intelligence - for the same reason, the three officials said.

The ODNI, headed by James Clapper, was established after the Sept. 11, 2001, attacks on the recommendation of the commission that investigated the attacks. The commission, which identified major intelligence failures, recommended the office's creation to improve coordination among U.S. intelligence agencies.

Director of National Intelligence (DNI) James Clapper testifies before a Senate Intelligence Committee hearing on "Worldwide threats to America and our allies" in Capitol Hill, Washington Thomson Reuters

In October, the U.S. government formally accused Russia of a campaign of cyber attacks against American political organizations ahead of the Nov. 8 presidential election. Democratic President Barack Obama has said he warned Russian President Vladimir Putin about consequences for the attacks.

Reports of the assessment by the CIA, which has not publicly disclosed its findings, have prompted congressional leaders to call for an investigation.

Obama last week ordered intelligence agencies to review the cyber attacks and foreign intervention in the presidential election and to deliver a report before he turns power over to Trump on Jan. 20.

The CIA assessed after the election that the attacks on political organizations were aimed at swaying the vote for Trump because the targeting of Republican organizations diminished toward the end of the summer and focused on Democratic groups, a senior U.S. official told Reuters on Friday.

Moreover, only materials filched from Democratic groups - such as emails stolen from John Podesta, the Clinton campaign chairman - were made public via WikiLeaks, the anti-secrecy organization, and other outlets, U.S. officials said.

'Thin reed'

The CIA conclusion was a "judgment based on the fact that Russian entities hacked both Democrats and Republicans and only the Democratic information was leaked," one of the three officials said on Monday.

"(It was) a thin reed upon which to base an analytical judgment," the official added.

Republican Senator John McCain said on Monday there was "no information" that Russian hacking of American political organizations was aimed at swaying the outcome of the election.

"It's obvious that the Russians hacked into our campaigns," McCain said. "But there is no information that they were intending to affect the outcome of our election and that's why we need a congressional investigation," he told Reuters.

Sen. John Kerry, D-Mass. listens at left as Sen. John McCain, R-Ariz., speak during a news conference on on Capitol Hill in Washington, Monday, Dec. 3, 2012, to urge Senate approval of an international agreement for protecting the rights of individuals with disabilities. AP

McCain questioned an assertion made on Sunday by Republican National Committee Chairman Reince Priebus, tapped by Trump to be his White House chief of staff, that there were no hacks of computers belonging to Republican organizations.

"Actually, because Mr. Priebus said that doesn't mean it's true," said McCain. "We need a thorough investigation of it, whether both (Democratic and Republican organizations) were hacked into, what the Russian intentions were. We cannot draw a conclusion yet. That's why we need a thorough investigation."

In an angry letter sent to ODNI chief Clapper on Monday, House Intelligence Committee Chairman Devin Nunes said he was “dismayed” that the top U.S. intelligence official had not informed the panel of the CIA’s analysis and the difference between its judgment and the FBI’s assessment.

Noting that Clapper in November testified that intelligence agencies lacked strong evidence linking Russian cyber attacks to the WikiLeaks disclosures, Nunes asked that Clapper, together with CIA and FBI counterparts, brief the panel by Friday on the latest intelligence assessment of Russian hacking during the election campaign.