WASHINGTON — President Donald Trump's lawyer has acknowledged that he received briefings from attorneys for Paul Manafort while Manafort was cooperating with Robert Mueller, an unusual development that legal experts say raises the specter of witness tampering and obstruction of justice.

Trump's chief defense lawyer, Rudy Giuliani, told the Associated Press that Manafort's attorneys had been briefing him, a fact first reported by The New York Times, to whom Giuliani also confirmed the briefings.

"They share with me the things that pertain to our part of the case," Giuliani told the AP. He declined to make the same acknowledgement to NBC News, but he did not dispute the reporting.

Some legal experts believe that the feeding of information by Manafort's lawyers to the Trump legal team could amount to obstruction of justice or witness tampering, if Manafort disclosed confidential information or the president's side discussed the possibility of a pardon. But that would depend on exactly what was said.

If Mueller decides to investigate, the Trump team could be in for an unpleasant surprise. Legal experts told NBC News the conversations among the lawyers would not be covered by attorney-client privilege — meaning Mueller in theory could haul Giuliani before the grand jury and force him to testify under oath about them.

"If you are trying to corruptly influence his testimony by dangling a pardon, that could be witness tampering," Daniel Goldman, a former federal prosecutor who is now an NBC News and MSNBC legal analyst, said on "Andrea Mitchell Reports."

Giuliani has said that the Trump and Manafort teams were part of a joint defense agreement under which they shared information. Such agreements are common among associates caught up in the same criminal investigation.

Rudy Giuliani, former mayor of New York, speaks with reporters at the White House on May 30, 2018. Bloomberg / Getty Images file

Typically, the lawyers party to those agreements can shield their conversations from the prying eyes of prosecutors under the guise of attorney-client privilege.

It can be written or oral, and lawyers in such an agreement in many cases may share information confident that it is protected under attorney-client privilege.

But as a matter of law, the experts say, a joint defense agreement can only exist between people who have a common legal interest. Once Manafort began cooperating with Mueller, he ceased to have a common interest with Trump, a subject of the investigation.

As an example, when former national security adviser Michael Flynn flipped, he notified Trump's lawyers that he was withdrawing from their joint defense agreement.

Once people involved in litigation no longer have a common interest — even if they say they do, courts have ruled — the conversations are not protected.

"Once Manafort signed up to cooperate with prosecutors, the 'common interest' he shared with Trump and others that let them shield their communications from prosecutors behind the attorney-client privilege evaporated," said Joyce Vance, a former U.S. attorney and NBC News analyst.

Goldman and fellow former U.S. attorney Mimi Rocah, also a NBC News analyst, agreed.

"Any information that was shared after this point isn't protected by their joint defense agreement," Vance said. "If I were Mueller I would be issuing subpoenas today to Rudy Giuliani and Kevin Downing, and anyone else involved in those communications, so they would have to explain under oath exactly what was conveyed."

There are practical reasons why this may not happen — Mueller may not want to pick this fight — but legally, it's possible, according to the experts.

Mueller said this week that Manafort violated the terms of his plea agreement by lying to prosecutors, but he didn't disclose the nature of the lies. Mueller said he would file a document spelling that out.

A judge, meanwhile, has scheduled a status conference for Friday in Washington. It's not clear whether Manafort's decision to allow his lawyers to feed information to the Trump team is part of the misconduct Mueller is alleging, but legal experts suspect as much.

"When Paul Manafort pleaded guilty, he signed a cooperation agreement with the government," said former federal prosecutor Chuck Rosenberg, an NBC News analyst. "He switched from the Red Sox to the Yankees. It would be like playing for the Yankees, and now giving the Red Sox scouting reports. You can't do that."

Joyce emphasized that the situation is extraordinary — and troubling — on multiple levels.

"The whole idea that we are talking about a president having a joint defense agreement with 32 other people — because they are all under criminal investigation — is not normal, and we can't let it become normal," she said.