Islamabad, May 31 (IANS): Celebrated Pakistani journalist Syed Saleem Shahzad, who went missing Sunday and was thought to have been seized by the ISI intelligence agency, was found tortured and murdered Tuesday, authorities and colleagues said.

The body of 40-year-old correspondent of Asia Times Online, whose writings on Islamist groups and on Pakistan and Afghanistan enjoyed a vast readership, was discovered in a canal in Mandi Bahauddin area of Punjab province, about 150 km southeast of Islamabad and about 10 km from where his car was traced, media reports said.

Local officials said the body bore visible marks of torture.

A doctor who conducted an autopsy said: "It is very disturbing for all of us. He was beaten to death, both his ribs were broken and marks of wounds were on his left side and on the legs."

The well-informed Shahzad, who also worked for the Italian news agency AKI, leaves behind his wife and three children - two sons aged 14 and seven and a daughter aged 12.

Prime Minister Syed Yusuf Raza Gilani expressed his "deep grief and sorrow" over Shahzad's death and ordered an inquiry into his kidnapping and murder, Associated Press of Pakistan reported.

Gilani vowed to bring murderers to justice "at every cost". The government, he said, strongly believed in the freedom of expression and considered media freedom an essential ingredient to strengthening democratic values and culture.

Shahzad went missing from Islamabad Sunday evening while going to the Dunya TV office to take part in a programme related to the terrorist attack May 22 on a major naval base in Karachi that left 10 security personnel and four militants dead.

Since then, his mobile phone was switched off.

The Hong Kong-based Asia Times Online said Shahzad had on several occasions been warned by ISI officials over articles they deemed to be detrimental to Pakistan's national interests or image.

Days before his disappearance, Shahzad authored an article for Asia Times that reported that the Al Qaeda's operational arm carried out the audacious attack on the Karachi naval base after the navy refused to free sailors who had been arrested for suspected Islamist links.

This article, some of his friends said, may have been linked to his abduction -- and eventual murder.

Tony Allison, the Editor of Asia Times Online, expressed his deep concern for what he described as "one of the most fearless journalists" with whom he had ever worked.

"We will bring the utmost pressure to bear on the authorities over this case. We at Asia Times Online express our deepest sympathies for Saleem's family," Allison said.

Earlier Tuesday, the Human Rights Commission of Pakistan voiced concern over Shahzad's disappearance and demanded that he be released unharmed.

It said that his abductors must be identified and brought to trial.

No one claimed responsibility for his abduction, but Human Rights Watch said it may have been carried out by the ISI, Pakistan's main and feared intelligence agency which is known to function as a state within a state.

Media reports also said that Shahzad's friends had reportedly received telephone calls from purported intelligence officials saying he would be released soon.

Giuseppe Marra, director of the Adnkronos Group, for which Shahzad worked as a correspondent in Islamabad since 2004, paid tributes to the murdered journalist.

"Today, our exceptional colleague and very dear friend Syed Saleem Shahzad concluded his marvellous and epic mission. We will never forget his phone calls and his illuminating analyses on social and cutural realities so different from our own," Marra said.

"No one will ever be able to kill our memories of an outstandingly intrepid and brave colleague," he said, adding Shahzad was a man "who viewed journalism as a supreme cultural mission of peace".

In November 2006, Shahzad had been kidnapped by the Taliban in Afghanistan's Helmand province where he was reporting.

He and another Pakistani journalist were held for a week on suspicion of spying, "tried" by their captors and released unharmed after they "confessed" to wrongdoing.

Earlier, the International Federation of Journalists released a statement saying it "urgently appeals to the government of Pakistan to order its security and police agencies to respond immediately to find" Shahzad.