The allegation by the top U.S. military officer that Pakistan's government sanctioned the killing of a journalist who wrote about the country's powerful security establishment was "extremely irresponsible," the Pakistani state-run news agency said.

The verbal sparring over the death of Saleem Shahzad has added even more strain to U.S.-Pakistani relations, which have teetered badly since the American raid that killed Osama bin Laden on May 2 in a northwest Pakistani army town.

Shahzad's tortured body was found in late May after he'd told friends he'd been threatened by Pakistan's Inter-Services Intelligence agency, a spy unit that is notorious for harassing reporters in a country considered one of the deadliest in the world for journalists.

The ISI has denied it had anything to do with killing Shahzad, but the suspicions have persisted and prompted unusual levels of public criticism of the spy agency. Shahzad's death also added to the pressure on the Pakistani military since the unilateral U.S. raid against the al-Qaida chief, which left it humiliated.

On Thursday, U.S. joint chiefs of staff chairman Adm. Mike Mullen said he believed the Pakistani government "sanctioned" Shahzad's killing. Although Mullen acknowledged he could not directly tie the killing to the ISI, he was the first U.S. official to make such a public allegation.

Mullen further said that the reported abuse of other journalists in Pakistan is not a good road for the government in Islamabad.

"It's a way to continue to, quite frankly, spiral in the wrong direction," said Mullen, who has devoted enormous time in the last four years to trying to improve relations with Pakistani leaders.

The state-run Associated Press of Pakistan issued a statement hours later in which an unnamed government spokesman called Mullen's allegations "extremely irresponsible" and said that it "will not help in investigating the issue."

The news agency, which acts as a government mouthpiece, often does not name the officials it quotes in reports.

The spokesman further noted that the Pakistani government has created a commission to investigate Shahzad's death and urged anyone who has anything to share on the subject at the "national or international level" to do so with the panel.

The 40-year-old Shahzad was a well-known journalist who wrote for the Asia Times Online and other publications. He regularly investigated sensitive topics, such as the alleged ties between militants and the state.

Pakistan was the deadliest country for journalists in 2010, with at least eight killed in the line of duty, according to the New York-based Committee to Protect Journalists. Six died in suicide attacks, the group said in a report late last year.

Despite the dangers, the media establishment in Pakistan has expanded rapidly over the past decade, and reporters here operate with freedoms denied in most developing countries. Still, many privately admit to getting occasional pressure from security and intelligence officials.