Assailants have killed three people, including a senior military officer, at a mosque frequented by minority Shia worshippers in Sargodha, a Pakistani city west of Lahore, according to local police.
Sectarian strife has been worsening in Pakistan, where Shias make up about 20 percent of the 180 million population.
"Brigadier Fazal Zahoor was shot by masked gunmen while taking part in a religious ritual at the mosque," Farooq Hasnaat, a police officer, said, adding the attack took place late on Sunday.
"The gunmen arrived on motorbikes and burst into the mosque. They identified the brigadier and shot and killed him, his brother Fazal Subhani and a third man called Mohammad Ayub."
The mosque is located in a military cantonment.
Hasnaat said Zahoor had received threats from the banned organisation Sipah-e-Sahaba, which says it want to expel Shias from Pakistan.
No one claimed responsibility for the attack.
More than 800 Shias have been killed in targeted attacks in Pakistan since the start of 2012, according to Human Rights Watch, the New York-based organisation.
Armed groups like Lashkar-e-Jhangvi say they are fighting for a Sunni theocracy and Shias should leave the country or be killed.