Jeff Sessions met twice last year with Russia's ambassador, but did not disclose those encounters during his confirmation hearing to become attorney general, the Washington Post reported.
The meetings happened when Sessions was still a senator, the newspaper reported on Thursday, citing Justice Department officials.
At his January 10 confirmation to become attorney general, Sessions was asked about possible communication between US President Donald Trump's campaign and Russian officials.
He did not mention the meetings which reportedly took place in July and September.
The private conversation between Sessions and Russian Ambassador Sergei Kislyak in September took place in the senator's office, at the height of what US intelligence officials say was a Russian cyber campaign to upend the US presidential race, the Post said.
Sessions, in a statement on Wednesday evening, denied ever meeting "with any Russian officials to discuss issues of the campaign".
"I have no idea what this allegation is about. It is false," the statement read.
Justice Department spokeswoman Sarah Isgur Flores confirmed the meetings took place, but defended Sessions.
"There was absolutely nothing misleading about his answer," she said, adding that as a sentaor last year Sessions had more than 25 conversations with foreign ambassadors.
"He was asked during the hearing about communications between Russia and the Trump campaign - not about meetings he took as a senator and a member of the Armed Services Committee," Flores told the Post.
The Kremlin said it was unaware of any meetings between the pair, but insisted any such encounters would have been routine.
"I don't know of any meetings, as the work of the ambassador involves holding as many meetings as possible, including with the executive and legislative authorities in the country they are in," Kremlin spokesman Dmitry Peskov said.
The previously undisclosed discussions could fuel new congressional calls for the appointment of a special counsel to investigate Russia's alleged role in the 2016 presidential election, the Post said.
READ MORE: US Senate approves Jeff Sessions as attorney general
The revelations about Sessions come one month after Trump's first national security adviser, Michael Flynn, was fired after it was reported that he discussed US sanctions on Russia with Kislyak before Trump took office.
Flynn later admitting to giving "incomplete information" to Vice President Mike Pence regarding the conversations.
As attorney general, Sessions oversees the Justice Department, including the Federal Bureau of Investigation, which have been leading investigations into Russian meddling and any links to Trump's associates.
When Sessions spoke with Kislyak, he was a senior member of the influential Senate Armed Services Committee as well as one of Trump's top foreign policy advisers, according to the Post.
Sessions played a prominent role supporting Trump after formally joining the campaign in February 2016.
Nancy Pelosi, the top Democrat in the House of Representatives, led calls for Sessions to quit.
"After lying under oath to Congress about his own communications with the Russian, the attorney general must resign," she said.
'I did not have communications with the Russians'
At his Judiciary Committee confirmation hearing in January, Sessions was asked by Democratic Senator Al Franken what he would do if he learned of any evidence that anyone affiliated with the Trump campaign communicated with the Russian government in the course of the 2016 campaign, the Post reported.
"I’m not aware of any of those activities," Sessions responded, according to the Post.
He added: "I have been called a surrogate at a time or two in that campaign and I did not have communications with the Russians."
Speaking to Al Jazeera from Washington, former associate US attorney general Bruce Fein told Al Jazeera: "Everyone knows Jeff Sessions was up to his ears in the Trump campaign, that's why he's attorney general now."
He said that Trump has made powerful enemies so far in his presidency, including the press, that were "out to get him".
"And there are many people in the intelligence community that are probably out to get Mr Trump too because he's derided them, [accused] them of doing things equivalent to the Nazis. So he's going to have an intelligence community that's looking for things," Fein added.
