The State Department joined the Hillary Clinton campaign Thursday in pushing back on Romanian hacker Guccifer’s claims that he successfully breached the former secretary of state’s personal server.

Marcel Lehel Lazar, who goes by the moniker "Guccifer," told Fox News that he accessed Clinton’s server “like twice,” claiming that for him, “it was easy.”

Asked about the claims at Thursday’s press briefing, State Department spokesman Mark Toner said he’s not aware of such an incident.

“We don’t have any reason to believe that it might be true,” he said.

At the same time, Toner repeatedly stressed he did not want to comment on the security of the server, citing ongoing investigations. Asked if he’s in a position to even know whether Lazar’s claims are true, Toner again said he did not want to comment.

The Clinton campaign issued a more definitive denial.

In response to Lazar’s claims, the Clinton campaign issued a statement Wednesday night saying, "There is absolutely no basis to believe the claims made by this criminal from his prison cell. In addition to the fact he offers no proof to support his claims, his descriptions of Secretary Clinton's server are inaccurate. It is unfathomable that he would have gained access to her emails and not leaked them the way he did to his other victims.”

However, Lazar provided extensive details about his hacking, in interviews from the Virginia jail where he is being held.

The 44-year-old Lazar said he first compromised Clinton confidant Sidney Blumenthal's AOL account, in March 2013, and used that as a stepping stone to the Clinton server. He said he accessed Clinton’s server “like twice,” though he described the contents as “not interest[ing]” to him at the time. 

“I was not paying attention. For me, it was not like the Hillary Clinton server, it was like an email server she and others were using with political voting stuff," Guccifer said.

The hacker spoke freely with Fox News from the detention center in Alexandria, Va., where he’s been held since his extradition to the U.S. on federal charges relating to other alleged cyber-crimes.

While Lazar's claims cannot be independently verified, three computer security specialists, including two former senior intelligence officials, said the process described is plausible and the Clinton server, now in FBI custody, may have an electronic record that would confirm or disprove Guccifer’s claims.

As recently as this week, Clinton said neither she nor her aides had been contacted by the FBI about the criminal investigation. Asked whether the server had been compromised by foreign hackers, she told MSNBC on Tuesday, “No, not at all.”

Recently extradited, Lazar faces trial Sept. 12 in the Eastern District of Virginia. He has pleaded not guilty to a nine-count federal indictment for his alleged hacking crimes in the U.S. Victims are not named in the indictment but reportedly include Colin Powell, a member of the Bush family and others including Blumenthal. 

Lazar also told Fox News that during his flight to the U.S. from Romania on or about March 31, he was accompanied by an FBI agent and a State Department official. He said he talked to the FBI and “wrote up eight pages of notes” during the flight, stressing that “I have nothing to hide.”

Lazar told Fox News that, while imprisoned in Romania where he was serving a seven-year sentence, he met several times with U.S. agents from the FBI, Secret Service and Treasury.

Presumptive Republican nominee Donald Trump reacted to the report Thursday, calling the claim by Guccifer "very serious."

"If it's true it’s very serious," Trump told Fox News' Bret Baier. "It’s not a question of him. It’s what she did and why did she do it?"

Fox News’ Catherine Herridge and Pamela Browne contributed to this report.