Three police officers were killed by unknown assailants in the Pakistani city of Faisalabad on Friday, police said on Saturday, in an incident that is being investigated as a possible attack by Islamic State (ISIS) group.

The three attackers, who escaped the scene, dropped copies of a pamphlet addressed to security officials which said a regional chapter of the Middle East-based militant group claimed responsibility for "recent attacks on security forces," a senior police official said.

It warned police against "protecting the un-Islamic system" of democracy and urged them to quit their jobs or face violent consequences, according to a copy of the pamphlet seen by Reuters.

Police said they were investigating "various leads" into the attack.

"The role of ISIS cannot be ruled out, though they don't have any known presence in Faisalabad," police chief Afzaal Kausar told Reuters.

He said similar attacks had taken place in Lahore and Islamabad during the past week, and police suspected the same group may be responsible.

Earlier this month, the government reasserted that there was "no organised presence" of ISIS in Pakistan after an intelligence chief warned a parliamentary panel that the group was a threat, and was coordinating with militant groups in the country.

The entry of ISIS into Pakistan would complicate the government's fight against indigenous Islamist militants fighting to overthrow it.

Late last year, a few breakaway factions of the Pakistani Taliban, which seeks to topple the government of Prime Minister Nawaz Sharif and set up a sharia state, declared allegiance to ISIS and ordered militants across the region to join its campaign to set up a global Islamic caliphate.

But in January, the Pakistani Taliban released a statement rejecting ISIS leader Abu Bakr al-Baghdadi's claim to be the global leader of all Muslims.