Michael Flynn, President Donald Trump's former national security adviser, took trips to Egypt and Israel in 2015, which have now become potential leverage in the ongoing federal investigation into U.S. President Donald Trump's ties to Russia.

The revelation comes a week after Flynn's lawyer told Trump's legal team they can no longer discuss the probe into Russian meddling in the U.S. election, indicating Flynn may be cooperating with the federal investigation led by Special Counsel Robert Mueller.

The Washington Post reported Monday that Flynn travelled to Egypt and Israel in 2015 in an effort to build more than two dozen nuclear plants in the Middle East. Flynn's trip was paid for by a U.S. company he was advising, but the nuclear plants were a project that involved "Russian interests."

Flynn's trips to the Middle East reportedly add pressure on him to cooperate with Mueller as he failed to disclose them - along with others that he is under investigation for.

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In previous comments to the Washington Post, Trump attorney Jay Sekulow was quoted as saying the move was "not entirely unexpected" and added: "No one should draw the conclusion that this means anything about General Flynn cooperating against the president."

Reuters was unable to reach Sekulow or Trump's other outside attorney, John Dowd, on Thursday. Flynn's lawyer and a spokesman for Mueller declined to comment.

The probe has hung over the White House since January, when U.S. intelligence agencies concluded that Russia interfered in the election to try to help Trump defeat Democrat Hillary Clinton by hacking and releasing embarrassing emails and disseminating propaganda via social media to discredit her.

Russia has denied interfering in the U.S. election and Trump has said there was no collusion.

The Times reported that Flynn's lawyers had been sharing information with Trump's legal team about the Mueller investigation. Citing four unnamed people involved in the case, the newspaper reported the cooperation agreement had ended.

Due to rules that aim to prevent conflicts of interest when lawyers represent clients, the move by Flynn's lawyers to stop communicating with Trump's lawyers indicated Flynn was now cooperating with Mueller, the Times said, although adding that in itself was not proof.

But the development has led Trump's lawyers to believe that Flynn has begun discussions with Mueller about cooperating, according to the Times.

Flynn served 24 days as Trump's national security adviser but was fired after it was discovered he had misrepresented his contacts with a Russian diplomat to Vice President Mike Pence.

Mueller's inquiry is looking into Flynn's paid work as a lobbyist for a Turkish businessman in 2016, in addition to contacts between Russian officials and Flynn and other Trump associates during and after the Nov. 8 presidential election, Reuters reported in June.

A lawyer for Flynn's son, Michael Flynn Jr., who worked with his father and is also being investigated by Mueller, according to a person familiar with the matter, declined to comment.