“If it is true, than the statement is extremely irresponsible," said a Pakistani official reacting to his comments adding, “It will not help in investigating the issue."
A judicial committee to look into Shahzad's death was set up by the government and any information “at national or international level" needed to be communicated with the commission.
“If any statement is issued other than this way, it will be considered an attempt to influence the proceeding of the Commission," said the spokesman.
“It seems that some elements are trying to use this issue against the elected democratic government and Pakistan," he added.
The remarks made by Mullen are sure to affect the ties between both the countries, which is already on bad terms after the death of terror head Osama Bin Laden in May by US Navy.
Mullen had earlier blamed the Pakistani government for approving the death of the Pakistani journalist.
"It's been reported recently and I haven't seen anything that would disabuse that report," Mullen had said earlier to reporters.
He said that he did not have any concrete evidence on the death of Shahzad but hinted at the role of Inter Service Intelligence (ISI) in his murder.
“It was sanctioned by the government, yeah," said Mullen to reporters from the Pentagon Press Association.
"I am hugely concerned about obviously his death. His isn't the first. For, whatever reason it has been used as a method historically. There are others certainly claims historically. I have seen Pakistani officials. I just gave them a room who deny it," he had said.