Pakistani Prime Minister Raja Pervez Ashraf has ordered an immediate operation against suspected terrorists three days after a huge bomb attack killed 87 people in the southwestern city of Quetta, an official said Tuesday.

Saturday's blast targeting Shiite Muslims in a crowded marketplace had also injured nearly 200 people, the bomb attack prompted countrywide protest and the relatives of the victims have refused to bury the dead unless the perpetrators are arrested.

The outlawed extremist group "Lashkar-e-Jhangvi" had claimed responsibility for the attack. The outfit, which is linked to the Taliban, routinely claims responsibility for attacks on Shiite Muslims.

In order to try to end protest of the Shiites in Quetta, the prime minister ordered operation to apprehend the culprits.

A spokesman of the prime minister's House said Ashraf has ordered an immediate launch of targeted operation aimed at eliminating those who are responsible for playing with lives of innocent civilians, and restoring peace and security in Quetta.

The spokesman further said that the prime minister is closely monitoring the situation in Quetta and is in constant touch with concerned authorities.

Officials said that the security forces conducted early Tuesday operation in parts of Quetta and killed four suspects in exchange of fire.

The exchange took place during a search operation, they said. A number of suspects were also arrested and weapons were recovered during the operation.

Meanwhile Chief Justice of Pakistan Iftikhar Muhammad Chaudhry, who had taken note of the attack, remarked on Tuesday that an operation cleanup should have been conducted against the Lashkar-e- Jhangvi (LeJ).