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"This is going to be really interesting because I have no idea what [Greenwald] is talking about," Hayden said. House Intelligence Committee Chairman Mike Rogers (R-Mich.) added, "Neither does he."

"Assassinations are forbidden by executive order," Hayden continued. "We don't do assassinations."

But he said the government does perform "targeted killings against enemy combatants" because the nation is at war.

The Obama administration has acknowledged using targeted killings, including against American citizens in certain cases, but the NSA's role in the strikes remains unclear.

Hayden made the comments minutes after joking that he had thought about adding Edward Snowden's name to a kill list. He noted that Snowden has been nominated for a European human rights award.

"I must admit in my darker moment over the past several months, I'd also thought of nominating Mr. Snowden, but it was for a different list," Hayden said.