Death toll from Indonesia school collapse rises to 45

The death toll from an Indonesian school collapse rose to 45 on Sunday, officials said, as rescuers recovered more victims buried under the rubble.

Part of the multi-storey building on Indonesia's Java island collapsed suddenly on Monday as students gathered for afternoon prayers.

Rescuers retrieved another 19 victims from the rubble on Sunday, National Search and Rescue Agency (Basarnas) operations director Yudhi Bramantyo said.

"According to our calculation, the total number of victims we have evacuated are 149, with 45 reported dead ... and 104 survivors," Yudhi told reporters Sunday, up from a toll of 37 earlier on Sunday and including several body parts.

Nanang Sigit, the head of the local search and rescue agency, confirmed the same death toll.

Yudhi said before the latest bodies were found on Sunday that at least 26 people were still missing.

Rescuers were still working around the clock in the rubble of the Islamic boarding school.

"We will continue to do our best," he said.

"In other words, we will probably extend the operation until we are sure that all victims... have been recovered."

Yudhi estimated that authorities would be able to clear the rubble by Tuesday.

The recovery operation was around "60 percent" complete, national disaster agency official Budi Irawan told reporters earlier on Sunday.

"Our hope is that by tomorrow everything will be levelled and we can determine the approximate number of victims who are in the rubble," Budi said in a livestreamed news conference.

Investigators have been looking into the cause of the collapse, but initial signs pointed to substandard construction, according to experts.