Director of National Intelligence Clapper listens to remarks by U.S. President Obama in McLean Thomson Reuters Director of National Intelligence James Clapper, speaking about the hack of Democratic Party emails, said on Thursday the U.S. intelligence community was not ready to "make the call on attribution" as to who was responsible.

The White House said earlier the FBI had not disclosed any information about who was behind the hack.

Cyber security experts and U.S. officials said on Monday there was evidence that Russia engineered the release of sensitive Democratic Party emails in order to influence the U.S. presidential election.

The emails were released by the anti-secrecy group WikiLeaks on the eve of the Democratic National Convention, and appeared to show that the Democratic National Committee favored Hillary Clinton over her chief rival, U.S. Senator Bernie Sanders.

Clapper, speaking at the Aspen Security Forum, acknowledged that "there’s just a few usual suspects out there" who might be responsible for the cyber intrusion, suggesting it was the work of a nation-state rather than an independent hacking group.

"We don’t know enough to ascribe motivation," he said. "Was this just to stir up trouble, or was this ultimately to try to influence an election? That’s a serious proposition."

Russian officials have dismissed the allegations of Moscow's involvement.

"It is so absurd it borders on total stupidity," said Kremlin spokesman Dmitry Peskov. "The Americans need to get to the bottom of what these emails are themselves and find out what it's all about."

Clapper said comments by Republican presidential nominee Donald Trump questioning U.S. alliances in Europe and Asia had rattled foreign intelligence and security officials.

While declining direct comment on Trump's remarks, he said: "Such statements, such rhetoric are very bothersome to our foreign interlocutors, our foreign partners. I hear that from counterparts, intelligence and security colleagues in many other countries. … It is a worry to them, it really is."

(Reporting by Warren Strobel; Writing by Yara Bayoumy; Editing by Steve Orlofsky and Peter Cooney)