(CNN) House Intelligence Committee Chairman Devin Nunes warned Sunday that he plans to urge lawmakers "this week" to hold Attorney General Jeff Sessions in contempt of Congress for failing to hand over classified materials related to the Russia investigation.

But the Justice Department informed Nunes three days ago -- on the deadline for responding to a subpoena from Nunes' committee -- that providing the information on a "specific individual" could pose grave implications for national security, according to a letter obtained by CNN.

"Disclosure of responsive information to such requests can risk severe consequences, including potential loss of human lives, damage to relationships with valued international partners, compromise of ongoing criminal investigations, and interference with intelligence activities," wrote Assistant Attorney General Stephen Boyd, who heads the Justice Department's Office of Legislative Affairs.

It was not immediately clear why Nunes has targeted Sessions. A source familiar with the matter said that the request falls squarely within Sessions' recusal from all materials related to the Russia investigation. Deputy Attorney General Rod Rosenstein has been fielding the document requests in his place.

Nunes has not described precisely what information he's seeking, but he said Sunday on "Fox and Friends" that it's "very important."

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