JAKARTA (Reuters) -Rescuers battling to extricate students from the rubble of an Islamic school that collapsed, killing three in Indonesia's province of East Java, faced a harder task on Wednesday after an earthquake that authorities fear packed the debris tighter.

Authorities said 91 people were listed as missing, with 100 evacuated and dozens injured after the collapse during students' late afternoon prayers in a mosque on the lower floor of a building whose upper floors were still under construction.

Tuesday's quake of magnitude 6.5 reduced the space for the people still trapped, complicating the task of rescue by narrowing the room for manoeuvre, said Emi Frizer, an official of Indonesia's search and rescue agency.

"How to hold on to the targets' lives while still having the same access -- that's going to take us a little longer," he said, adding that searchers had to be careful not to injure victims' limbs during their rescue.

"If the space was initially 50 cm (20 inches) high, it caved in to 10 cm (4 inches), and we fear it impacts the constriction of the victims," said Mohammad Syafii, the chief of the search and rescue agency.

Disaster officials said three people died in the collapse of the boarding school in the town of Sidoarjo, about 780 km (480 miles) east of Jakarta, the capital.

The earthquake that struck the region of Sumenep, about 200 km (124 miles) from the collapse site, injured three people and damaged dozens of homes, Indonesia's disaster mitigation agency said on Wednesday.

Rescuers detected signs of life that helped them narrow the search for survivors, Emi said.

Authorities have said the building's foundations could not support construction work on its upper levels, leading to the collapse.

"This is all foundational failure," Emi added.

An excavator and a crane were at the site to help rescuers shift rubble, but local official Nanang Sigit ruled out their use for fear it could set off a wider collapse.

(Reporting by Stanley Widianto; Editing by Clarence Fernandez)

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