(CNN) FBI Director Christopher Wray underscored the importance of protecting the identity of human sources in testimony Wednesday amid an ongoing battle with House Intelligence Committee Chairman Devin Nunes and other House Republicans over the disclosure of a document the Justice Department says would endanger a source.

"The day that we can't protect human sources is the day the American people start becoming less safe," Wray told members of the Senate Appropriations Committee. "Human sources in particular who put themselves at great risk to work with us and with our foreign partners have to be able to trust that we're going to protect their identities and in many cases their lives and the lives of their families."

Wray said he respected Congress' oversight duties and "we welcome the tough questions given our responsibilities" but defended the pace of the bureau's document production, as the FBI and Justice Department have faced subpoenas and criticism from House Republicans.

In March, Wray said he doubled the number of staff who were reviewing documents requested as part of one Hill probe.

"We actually have been dealing with quite a number of requests, some of them almost unprecedented in their volume from a number of different committees," Wray said Wednesday. "We're chugging away, making progress."

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