US Attorney-General Jeff Sessions is facing calls to resign.

The Washington Post reported that Sessions had failed to disclose meetings with Sergey Kislyak, Russia's ambassador, while he was still a US senator and supporter of Donald Trump's campaign.

Sessions didn't mention any of his meetings during his confirmation hearings.

He said he knew of no contacts between Mr Trump and Russia.

US House Democratic leader Nancy Pelosi called on Sessions to resign over the issue.

Senate Democratic leader Chuck Schumer also demanded Jeff Sessions stand down.

He said in the least, there's enough doubt swirling to warrant an investigation.

"There's nothing inappropriate with the senator meeting with the Russian ambassador. There is something very inappropriate to dramatically mislead Congress.

"The Attorney General is the chief law enforcement officer of the land, and already his integrity and independence has been questioned. It would be better for the country if he'd resign."

Democratic senator Al Franken said it's simply not good enough.

"He should have said so. He should have said 'I met with the Russian ambassador a couple of times, but we didn't discuss the campaign."

The question of how much contact there was between the campaign of President Donald Trump and Russia prior to his election has dominated the early days of his presidency along with allegations from US intelligence officials that Moscow ran an influence campaign to try to sway the election's outcome.

Russia categorically denies meddling and Russian officials say the issue is being deliberately used by Trump's opponents to derail chances of a swift warming in US-Russia ties.

"The embassy doesn't comment on numerous contacts with local partners, which occur on a daily basis in line with diplomatic practice," Russian embassy spokesman Nikolai Lakhonin told Russia's Interfax news agency, when asked to comment on meetings between Sessions and Kislyak.

Sessions has denied discussing details of the US presidential campaign with Russian officials.

Trump's first national security adviser, Michael Flynn, was fired last month after he discussed US sanctions on Russia with Kislyak before Trump took office and misled Vice President Mike Pence about the conversations.

Michael McFaul, a former US ambassador to Russia under Barack Obama who has himself met Kislyak, said he was bemused by what he said was the Trump administration's secrecy about its contacts with Russian officials.

"I just don't get why Trump folks are so secretive about meetings/calls with Russians. If nothing to hide, just tell the truth!," McFaul said on social media.

Alexey Pushkov, a Russian senator, said the uproar over Sessions showed how paranoid US politicians had become about any links with his country.

"Almost the entire US elite is, it turns out, linked to Russia. Including the attorney general," Pushkov wrote on social media. "Paranoia knows no bounds."