The top-ranking U.S. military officer says elements within the Pakistani government apparently “sanctioned” the killing of a journalist in late May, but he could not tie the death to the country's intelligence service.

Admiral Mike Mullen says he has not seen anything to counter reports that Islamabad approved the beating death of Pakistani reporter Saleem Shahzad.

An unidentified Pakistani government spokesman told the state-run Associated Press of Pakistan that Mullen's statements are “extremely irresponsible.” He said Pakistan has established an independent panel to investigate Shahzad's death.

Pakistani's powerful Inter-Services Intelligence agency has dismissed allegations of its involvement as “baseless.”

Shahzad, who worked for the Hong Kong-based Asia Times Online, disappeared from Pakistan's capital Islamabad on May 29. His body was found in a canal two days later bearing signs of torture.

Before his death, Shahzad had been investigating alleged ties between Pakistan's military and Islamist groups. He reported that al-Qaida militants had attacked a Pakistani naval base after talks failed to secure the release of two naval officials arrested on suspicion of links to the global terror network.

Mullen acknowledged U.S. relations with Pakistan are “under extraordinary pressure.”

He said Islamabad's already fragile bond with Washington became severely strained when U.S. special operations forces killed al-Qaida leader Osama bin Laden in a raid on his compound deep into Pakistan in May.