House Minority Leader Nancy Pelosi (D-Calif.) said Monday that President Trump could be putting a person or operation in jeopardy if he disclosed classified information to Russian diplomats, calling it a "serious matter."

The Washington Post reported Monday evening that Trump shared highly classified intelligence about the Islamic State in Iraq and Syria (ISIS) with Russian officials last week. A U.S. partner, who served as the source for the intelligence, did not give the president permission to disclose the information with the Kremlin officials.

"This is code-word source 'highly classified information' revealed to an adversarial country," Pelosi said at a special CNN town hall with host Chris Cuomo, referring to the Russians.

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Although Trump was within his rights as president to declassify information at his discretion, his disclosure could possible burn the relationship the U.S. has with an intelligence source that has rare access and insights into ISIS’s inner workings, she said.

A U.S. official familiar with the meeting told the Post that Trump “revealed more information to the Russian ambassador than we have shared with our own allies."

Pelosi said Trump's disclosure could possibly allow the Russians to trace the "source or method."

"Even if the president didn't reveal a source or method ... what he did reveal could be traced very directly to a source and method," she said.

The California lawmaker said this could endanger the source and potentially undermine an important operation.

"It endangers the person or persons, the activity. It could undermine an operation that could be saving lives. And it undermines the trust that we would have with our allies," she added, saying other countries might lose trust in their relationship with the U.S. because they don't expect the president to share such sensitive information.

Pelosi also warned that now ISIS may be aware it is the target, which could lead it to push up the timeline for plans against the U.S.

"You never want the target to know they're the target. Or that you are on to them in a certain way. That has accelerated their actions in the past. That's another danger," she continued.

The president met with Russian Ambassador Sergey Kislyak and Russian Foreign Minister Sergey Lavrov at the White House last week.