Rescuers in East Java, Indonesia are racing to find dozens of people still believed trapped in the rubble of a school building that collapsed on Monday.

At least four students were killed and about 100 injured, some critically, after the two-storey Al-Khoziny Islamic Boarding School in the town of Sidoarjo caved in.

Hundreds of students, most of them teenage boys, had gathered to pray in the building when it gave way.

Authorities on Wednesday said crying and shouting could still be heard from under the rubble, while anxious relatives who had camped out at the school overnight awaited news of their loved ones.

Mohammad Syafii, head of the search and rescue agency Basarnas, told reporters there had been “confusion” regarding the number of people trapped in the building – but added that their “focus now is only on the rescue operation”.

Of the two students pulled out from the rubble on Wednesday – one had died and the other was being treated, Mr Syafii said.

Thirteen people have been freed from the debris since rescue efforts began on Monday, according to officials – but two died in hospital on Tuesday.

Two extra floors were being added to the building, but it could not support the weight of the construction on its unstable foundations, the disaster mitigation agency has said.

Sidoarjo town’s regent says the school’s management had not obtained permits to add more floors to the building.

Earlier, rescuers said one trapped victim was able to respond directly to them – and they were constructing a tunnel underneath the building to reach him.

The situation remains precarious for survivors trapped in rubble, however, with authorities warning of further collapse. An earthquake offshore overnight halted rescue work briefly, AFP news agency reported.

Mudji Irmawan, a civil engineering expert from the Sepuluh Nopember Institute of Technology who is involved in assessing the collapsed structure, told BBC Indonesia rescuers must be careful when removing the wreckage amid fears of destabilising the debris and causing further collapse.

Mr Syafii, the search and rescue chief, further noted that while rescuers help lift the concrete slabs and open up access, “shifting the slabs may also endanger the lives of survivors still trapped beneath the rubble”.

“When we excavate, there is a risk of small landslides on both sides,” Mr Syafii said. “With the current construction, with this rubble, even a single vibration can cause other impacts.”

Rescuers have to pass through underground culverts to reach survivors and only have about 60cm of access, he added.

Rescuers are racing against time to reach survivors during the first 72 hours of the disaster, when survival chances are highest.

“It is possible that we can detect life within this golden timeframe, allowing us to rescue the victims,” Mr Syafii said, though he added that with fluids and vitamins, people could live longer.

One survivor, 13-year-old Muhammad Rijalul Qoib, described the moments leading up to the building’s collapse.

“Many, perhaps hundreds of people, were about to pray,” he recalled. “At that moment, I heard the sound of falling rocks. The sound continued, and it got louder and louder.”

Rijalul immediately ran outside but was struck by falling debris from the roof.

“I wanted to run [from the prayer room] and then the roof hit my face,” he told local news outlet Detik.

He managed to survive by climbing out of the rubble, with the help of someone who pointed him toward an escape route.

Girls at the school were praying in another part of the building and also managed to escape, reports said.

Many boys were less fortunate. The first to be confirmed dead was Maulana Alvan Ibrahimavic, who had recently graduated from elementary school and enrolled at Al-Khoziny about four months ago.

Police escorted his body to the funeral home on Monday night, the police chief of Blega district – the student’s hometown – told local media.