Desperate parents on Wednesday demanded Indonesian officials speed up efforts to find dozens of children missing in the collapse of a school, with rescuers detecting signs of life under the rubble.

An estimated 91 people are believed to be in the ruins of the multi-storey school on the island of Java that gave way suddenly on Monday, as students had gathered for afternoon prayers.

At least three people were killed in the collapse, but based on school records "91 people are suspected to be buried," National Disaster and Mitigation Agency spokesperson Abdul Muhari said in a statement late Tuesday.

On Wednesday, tearful parents milled fearfully around the ruins, awaiting news of their children.

"We believe our children might still be alive because they were crying for help," said father Abdul Hanan, whose 14-year-old son is missing.

"The rescue operation must be accelerated," the 45-year-old told AFP.

"We are racing against time now," he added, bursting into tears.

Nearby, Dewi Sulistiana was awaiting news of her 14-year-old son. They were last in touch on Sunday. The boarding school limits student access to cellphones.

She rushed to the scene from Surabaya, several dozen kilometres (miles) away, after hearing about the collapse.

"I have been here for days. I cried thinking about my son," she told AFP.

"Why is taking so long to find him, why is the search so slow? I haven't had any updates, so I just wait."

- Tunnels and drones -

The rescue operation is complex, said Mohammad Syafi'i, head of the National Search and Rescue Agency.

"If vibration happens in one spot, it could affect other places. So now, to reach the spot where the victims are, we have to dig an underground tunnel," he told reporters.

Digging itself poses challenges, including possible landslides. And any tunnel will only provide an access route around 60 centimetres (about 23 inches) wide because of the structure's concrete columns.

Technology including thermal-sensing drones is being used to locate survivors and the deceased as the 72-hour "golden period" for best survival chances nears its end.

AFP saw rescuers in orange uniforms appearing to snake cameras under the rubble to hunt for traces of survivors.

So far, signs of life have been detected in seven areas, said Emi Freezer, of the National Search and Rescue Agency.

Water and food is being sent in, but access is through a single point, he said.

"The main structure has totally collapsed."

Complicating the operation, an earthquake struck offshore overnight, briefly halting the search.

Around the ruins, local charitable organisations have set up posts offering families food and drink.

The school collapse was so violent it sent tremors across the neighbourhood, said local resident Ani.

"I felt a vibration and then I heard a noise. I immediately ran to save myself. I didn't realise at first it was a building collapse," the grocery stall owner told AFP.

"Many people then also ran to save themselves," she added.

Investigations into the cause of the collapse are ongoing, but initial signs point to structural issues and construction that did not meet building standards, experts said.

The building collapsed after its foundation pillars failed to support the weight of new construction on the fourth floor of the school, said the national disaster management agency spokesman.

Lax construction standards have raised widespread concerns about building safety in Indonesia, where it is common to leave structures -- particularly houses -- partially completed, allowing owners to add extra floors later when their budgets permit.

Earlier this month, at least three people were killed and dozens injured when a building hosting a prayer recital collapsed in West Java province.

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