SAN SALVADOR, El Salvador—Attorney General William Barr denied he is standing in the way of special counsel Robert Mueller’s testimony before Congress, after the chairman of the House panel seeking his appearance accused the Justice Department of being unwilling to set a date.

“It’s Bob’s call whether he wants to testify,” Mr. Barr told The Wall Street Journal Wednesday, en route to El Salvador, a trip focused on increasing international cooperation against the violent street gang MS-13, which has roots in both Central America...