The US Senate's top Republican broke with President-elect Donald Trump on Monday over whether Russian hacking during the US election merited closer scrutiny, a fissure between Trump and his party that appeared to grow as lawmakers pressed for a special investigation into the matter.

The divide raised the possibility of enduring clashes between Trump and Republicans during his presidency over how to handle Russian President Vladimir Putin, a leader long viewed by many in the party as a calculating, untrustworthy foe but whom Trump has repeatedly praised for his leadership.

"The Russians are not our friends," Senate Majority Leader Mitch McConnell told a news conference.

Calls on Sunday by two leading Republican foreign policy voices, Senators John McCain and Lindsey Graham, to investigate Russia's hacking were buoyed on Monday by McConnell, who said Russia's involvement needed further investigation.

"Any foreign breach of our cybersecurity measures is disturbing, and I strongly condemn any such efforts," McConnell said. "This simply cannot be a partisan issue."

McConnell said it "defies belief" that Republicans would be reluctant to investigate Russian actions.

McConnell's remarks contrasted with those of Trump and his staff, who scoffed at reports that the CIA had concluded the hacks and leaks of Democratic emails were carried out with the goal of helping Trump defeat Democrat Hillary Clinton. Trump said the conclusion was "ridiculous".

Some lawmakers have called for a special committee to investigate the hacking, but McConnell did not back that idea, saying he has confidence in the Senate Intelligence and Armed Services committees.

It was not clear how the House of Representatives would respond to the hacking and calls for investigations. House committees have not announced plans for hearings, and Republican House Speaker Paul Ryan issued a statement criticizing Russia but blasting what he termed "exploiting the work of our intelligence committee for partisan purposes".

Though congressional Republicans support a probe of Russia's involvement in the election, they have shied away from agreeing with the CIA's assessment that the hacks were deliberately carried out to undermine Clinton.

"It's obvious that the Russians hacked into our campaigns," McCain said on Monday in an interview with Reuters.

"But there is no information that they were intending to affect the outcome of the election, and that's why we need a congressional investigation."