Sessions on Tuesday told a House committee he had "no reason to doubt" the accounts of women who have accused Moore of groping or pursuing romantic relationships with them when they were teens.
Sessions also hinted that the Justice Department might look into allegations against the besieged Republican Alabama Senate candidate.
"We will evaluate every case as to whether or not it should be investigated," Sessions told members of the House Judiciary Committee.
"These allegations are credible," Ryan, R-Wis., told reporters on Tuesday. "If he cares about the values and people he claims to care about then he should step aside."
Later Tuesday, Politico, citing a senior Republican Party official, reported that the Republican National Committee would back off its support for Moore and withdraw from a joint fundraising agreement it has with the Senate candidate. Politico also said the RNC would cancel field operations in support of Moore it had planned for Dec. 12, the day of the election.
Sessions' and Ryan's comments added to a growing chorus of Republicans who have called on Moore to step aside in his bid to fill the seat Sessions vacated earlier this year when he became attorney general in President Donald Trump's administration. Sessions had held the seat since 1997.
Sen. Cory Gardner of Colorado, who chairs the Senate GOP's campaign organization, has said that, if Moore is elected, senators should expel him "because he does not meet the ethical and moral requirements" of the Senate.
Such an expulsion would require a two-thirds majority, which means 19 Republicans would need to join the 46 Democratic and two independent senators to successfully block Moore from being seated.
As of Tuesday, some 15 GOP senators, along with other several other prominent party members, have said that Moore should withdraw from the race, saying, in effect, that the series of allegations are more credible than Moore's repeated, vehement denials.
Other GOP senators have said Moore should step aside "if the allegations are true." Six have not commented on the allegations.
When she was 16, Nelson said, Moore offered to drive her home from an after-school job at a restaurant where he was a regular customer. Instead, she said, he parked behind the restaurant, locked the door to keep her inside, and attempted to force her to perform oral sex on him.
Moore denied the accusations. "I can tell you without hesitation this is absolutely false. I never did what she said I did. I don't even know the woman," he said.
Moore is showing no sign of quitting the race and has denied multiple allegations. His campaign sent out fundraising emails emphasizing his stance as a maverick battling establishment politicians.
In one email sent Tuesday, Moore referred to Senate Majority LeaderMitch McConnell, R-Ky., and GOP Sens. John McCain of Arizona and Lindsey Graham of South Carolina as "the forces of evil who are attempting to rip and tear apart me and my campaign with their filthy lies and disgusting attacks."