ISLAMABAD, Pakistan (CNN) -- Pakistani forces fired at a fuel tank inside a controversial mosque early Saturday, causing an explosion heard across the capital, intelligence sources said.
A religious student walks with her father past soldiers after being released by authorities near the Red Mosque.
The fuel tank had been used for running generators in the besieged mosque, the sources said. There were no immediate reports of casualties.
Shortly after the blast, gunfire erupted again between the radical Islamic students inside the Lal Masjid, or Red Mosque, and the Pakistani security forces outside. The four-day siege has resulted in at least 26 deaths.
The latest confirmed deaths were two students who tried to surrender Friday but were shot dead by other students, intelligence sources said. The sources gave no additional details of how the shootings occurred.
The violence began Tuesday when about 150 militant students attacked a police checkpoint close to the mosque. Police fired tear gas, and the students fought back with sticks and guns and took refuge in the mosque. Watch what happened at the mosque
Tensions have been simmering between police and mosque students, who are blamed for a recent string of kidnappings of civilians, Chinese nationals and Pakistani police. The government has been investigating the activities of the mosque, whose students are demanding sharia, or Islamic law, be instituted in Islamabad.
More than 1,200 students have surrendered, but hundreds remain holed up inside.
Maulana Abdul Aziz, the top cleric of the Red Mosque, called on the students to surrender Thursday, a day after he was arrested trying to slip out of the mosque disguised in a burqa -- the head-to-toe covering worn by some Muslim women. E-mail to a friend
Syed Mohsin Naqvi contributed to this report.