Gunmen disguised as police officers stormed a hospital in Lahore where the injured from last Friday's massacre of 93 members of a Muslim sect had been receiving treatment, killing up to 12 people.
The attack on Jinnah Hospital in the heart of Lahore was an abortive attempt by militants to free a wounded cohort who had been arrested and was being treated there, Punjab government officials said.
It came just three days after two teams of gunmen attacked two mosques belonging to the Ahmedi sect of Muslims, when 93 people were killed in the biggest ever attack on the long-persecuted minority group.
Moments before midnight, four gunmen stormed into the hospital, announcing their entry with a rapid burst of gunfire. It is unclear how many innocent people were killed in the attack. The local police's own figures vary from six to 12.
"They barged into the hospital building and opened indiscriminate fire," said Javed Ikram, the chief executive of Jinnah Hospital.
During the siege, the attackers briefly took several patients hostage. After the Punjab police arrived at the scene, one of the gunmen went to the roof of the hospital and fired at their ranks. Four policemen were among the dead. The attackers had apparently been intent on releasing or killing a terrorist suspect referred to only as "Moaz". After the shootout with the police, the gunmen managed to escape, with one of them injured.
Last Friday's attack on the vulnerable Ahmedi community has set off alarm bells about the growing militant threat emerging from within Pakistan's largest and wealthiest province, Punjab. Lahore is the capital of Punjab province.
The Pakistani Taliban claimed responsibility for last Friday's attacks. While the group received training in the tribal area of North Waziristan along the Afghan border, a notorious sanctuary for the Pakistani Taliban and al-Qa'ida, one of the men arrested came from Rahimyar Khan, a town in southern Punjab.
As sectarian attacks in Pakistan escalate, there has been an intensifying focus on the southern part of the province, where hardline Sunni militant groups have long maintained a strong presence and the heaviest concentration of madrassahs in the country.
On Sunday, Interior Minister Rehman Malik said that the government is prepared to take action against these groups. "There will be an operation in south Punjab on the pattern of tribal areas," he was quoted as saying by local media.
Mr Malik added that Punjab's most notorious groups, that once started life attacking Pakistan's Shia community, have now become an integral part of the Taliban and al-Qa'ida's network in the country. The Sipah-e-Sahaba Pakistan (SSP), Jaish-e-Muhammad and Lashkar-e-Jhangvi have been banned as terrorist groups in Pakistan since 2002. But in recent years, they have resumed their activities, operating under different names and with a number of sub or splinter groups.