President Donald Trump's previously undisclosed conversation with Russian President Vladimir Putin is a "disturbing development," Rep. Adam Schiff, D-Calif., said Tuesday on MSNBC.

"It's certainly very possible that that third meeting where you don't have the secretary of State or even the Russia foreign minister present, the president may have betrayed – I don't mean in terms of the country – but betrayed his true interest where his red lines might be, what his amenability might be to giving back Russian properties," Schiff told "Hardball with Chris Matthews."

Trump reportedly spoke with Putin for about an hour in Hamburg, Germany, at the heads of state dinner. Putin's translator was the third person in attendance.

"We have no idea what was discussed," Schiff said. "We do know it was a long meeting and it was a private meeting where only Putin had someone there to confirm what was being discussed. So, deeply troubling, not in our national interest in any way, shape or form. Very much I think in Putin’s interest and yet another disturbing development."