(CNN) The House Energy and Commerce Committee says Health and Human Services Secretary Alex Azar has declined a request to appear before the committee to discuss family separations at the US southern border.

Chairman Frank Pallone, a New Jersey Democrat, personally requested that Azar testify before the committee, but the secretary's office declined the request on Tuesday, according to the committee spokesman.

"One way or another," Azar will have to testify before the Energy and Commerce Committee, Pallone said in a statement. He called Azar's denial of the request to testify "unacceptable" and said it is "outrageous" that Azar has yet to appear before Congress for a hearing on the controversial policy.

Between April 19 and May 31 last year more than 2,600 children were separated from their parents at the southern border , after the Trump administration ordered federal prosecutors to pursue criminal charges against all migrants who crossed the border unlawfully. The controversial policy was ultimately reversed in June with an executive order.

A new inspector general report released on Thursday found that "thousands" more children had been separated than previously reported by the government.

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