Islamabad, Pakistan (CNN) -- India expressed "great anguish" on Thursday over the death of one of its citizens following an attack in a Pakistani jail, calling for an investigation into what happened.
"This was, put simply, the killing of our citizen while in the custody of Pakistan jail authorities," the Indian Ministry of External Affairs said in a statement about the death of Sarabjit Singh, who had been held in a jail in the northeastern city of Lahore, near the Indian border.
Pakistan's Foreign Minister Arif Nizami told CNN that Singh had been convicted for espionage in Pakistan for his involvement in terror attacks in 1991. He was on death row for that crime. Singh's family has maintained his innocence.
Indian Prime Minister Manmohan Singh described Sarabjit Singh as "a brave son of India who bore his tribulations with valiant fortitude."
"The criminals responsible for the barbaric and murderous attack on him must be brought to justice," the prime minister said in a statement.
The Pakistani Ministry of Foreign Affairs said Thursday that Sarabjit Singh died of cardiac arrest in a Lahore hospital where he had been in a coma for the past few days after being hurt in "a scuffle with fellow inmates" at the jail last week.
Islamabad said it had been "providing all assistance to the family of Sarabjit Singh as well as to the Indian authorities since the occurrence of this unfortunate incident."
It said it would hand over his body to the Indian High Commission in Pakistan as soon as possible. Indian authorities had requested the return of his remains for burial.
His death puts a fresh strain on the delicate relations between India and Pakistan, who have fought several wars since their independence from British colonial rule more than 65 years ago.
Ties between New Delhi and Islamabad have improved since tensions flared in the aftermath of the 2008 terrorist attacks in Mumbai, for which India blames the Pakistan-based group Lashkar-e-Tayyiba.
But the fragile nature of their relationship was underscored by small-scale clashes between the two sides in January in the disputed territory of Kashmir.
"The shocking attack on Shri Sarabjit Singh highlights the need for concerted action by Pakistan to safeguard Indians in Pakistani jails," the Indian Ministry of External Affairs said in its statement Thursday.
CNN's Shaan Khan reported from Islamabad, and Jethro Mullen reported and wrote from Hong Kong.