The top Democrat on the House committee probing Russia's efforts to interfere with the U.S. election on Thursday accused FBI Director James Comey of withholding crucial information from lawmakers.

"I would say at this point we know less than a fraction of what the FBI knows," Rep. Adam Schiff, D-Calif., told reporters Thursday after a briefing with Comey at the Capitol.

"I appreciate we had a long briefing and testimony from the director today, but in order for us to do our investigation in a thorough and credible way, we're going to need the FBI to fully cooperate, to be willing to tell us the length and breadth of any counterintelligence investigations they are conducting," Schiff said. "At this point, the director was not willing to do that."

Schiff said during the briefing, which lasted more than three hours, there were areas of the investigation Comey "walled off" from discussions with lawmakers.

"Those walls are going to have to come down if we are going to do our job," he said. "And we're better off getting that through the voluntary cooperation of the FBI than having to contemplate whether we need to subpoena the FBI."

CNN reported that the committee chairman, Rep. Devin Nunes, shook his head and said "no" to a question about whether a special prosecutor was needed. The network said the California Republican also indicated that he has seen no evidence of contacts between Russian officials and the Trump campaign.

Even as connections have been revealed between Russian officials and several members of President Donald Trump's administration, GOP leaders in Congress have resisted calls to form a special committee to investigate.

But with the resignation last month of Trump's national security adviser, Michael Flynn, over his contacts with Russian Ambassador Sergey Kislyak, some Republicans joined Democrats in pushing for an expanded probe. New questions raised this week about Attorney General Jeff Sessions' meetings with Kislyak during the presidential campaign last year returned the issue to the spotlight.

Sen. Lindsey Graham is among those Republicans who have pushed the administration to take a hard line against Moscow and urged his colleagues in Congress as well as the FBI to investigate aggressively.

On Thursday, Graham, like Schiff, expressed frustration that he had been kept in the dark on crucial aspects of the agency's investigation.

"If there is an investigation at the FBI about Trump, Russia campaign activities, I want to know about it," Graham said.

"I'm tired of reading about it in the paper. The FBI needs to let us know if there is or there isn't," he said. "If there's not one they need to come forward and say so."