Indonesia’s geophysics agency lifted a tsunami warning 34 minutes after issuing it following a major earthquake that sent huge waves crashing into the north-eastern coast of Sulawesi island.

The 7.5-magnitude quake and tsunami, which hit the city of Palu, about 1,500km (940 miles) from Jakarta and further along the coastline, has killed at least 420 people. Officials said on Saturday the death toll was likely to rise significantly.

Hundreds of people had gathered for a festival on the beach in Palu on Friday when waves as high as six metres smashed onshore at dusk, sweeping many to their death.

The geophysics agency, BMKG, faced criticism on Saturday on social media, with many debating the timing of the tsunami warning.

The agency said it followed standard operating procedure and made the call to “end” the warning based on data available from the closest tidal sensor, around 200km from Palu.

“We have no observation data at Palu. So we had to use the data we had and make a call based on that,” said Rahmat Triyono, head of the earthquakes and tsunami centre at BMKG.

He said the closest tide gauge, which measures changes in the sea level, only recorded an “insignificant” 6cm wave and did not account for the giant waves near Palu.

“If we had a tide gauge or proper data in Palu, of course it would have been better. This is something we must evaluate for the future,” said Triyono.

It was not clear whether the tsunami, which officials say hammered Palu and the surrounding area at extremely high speeds measuring in the hundreds of kilometres per hour, occurred before or after the warning had been lifted.

“Based on the videos circulating on social media, we estimate the tsunami happened before the warning officially ended,” Triyono said.

Baptiste Gombert, a geophysics researcher at the University of Oxford, said it was “surprising” the quake had generated a tsunami.

Friday’s quake was recorded as a “strike-slip” event where neighbouring tectonic plates move horizontally against each other, rather than vertically, which is what usually generates a tsunami.

“There is some speculation that there was a landslide under the sea, which displaced a lot of water and caused the tsunami,” he said, adding the Palu’s narrow bay may have concentrated the force of the waves as they moved toward the shore.

Sutopo Purwo Nugroho, a spokesman for the disaster agency, told reporters his team had been “preparing to send public warnings that were easy to understand” when the tsunami warning was “suddenly ended”.

The communications ministry said repeated warnings were sent out to residents via text message, but Nugroho said the quake had brought down the area’s power and communications lines and there were no sirens along the coast.

Indonesians took to social media to question the BMKG’s move to lift the tsunami warning and a failure to release information in a timely manner.

“So upset ... the warning was lifted ... although a tsunami happened,” said Twitter user @zanoguccy in a direct message to BMKG.