According to authorities, two foreign visitors went lost when ice collapsed during their excursion to a glacier, and rescuers in south Iceland are attempting to locate them by hand.

On Sunday, shortly before 15:00 (Iceland observes GMT), first responders received a call regarding the Breidamerkurjökull glacier collapse.

Twenty-five people, accompanied by a guide, were touring an ice cave when the two went missing. The incident resulted in the serious injuries of two persons.

"The conditions are very difficult on the ground," said local police chief Sveinn Kristján Rúnarsson. "It's in the glacier. It's hard to get equipment there... It's bad. Everything is being done by hand."

Local news outlets reported that 200 people were working on the rescue operation which would continue into the night.

Speaking on Icelandic TV, Chief Superintendent Rúnarsson said police had been unable to contact the two missing people.

"So we don't know what the situation is but two others who were freed are badly injured," he added.

While the conditions were "difficult", the weather was "fair", he said.

Confirming that all those involved were foreign tourists, he said there was nothing to suggest that the trip to the cave should not have taken place.

"Ice cave tours happen almost the whole year," he said.

"These are experienced and powerful mountain guides who run these trips. It's always possible to be unlucky. I trust these people to assess the situation - when it's safe or not safe to go, and good work has been done there over time. This is a living land, so anything can happen."

The police chief was quoted as saying that people had been standing in a ravine between cave mouths when an ice wall collapsed.

Breidamerkurjökull is a glacial tongue which extends from the Vatnajökull glacier to the Jökulsárlón lagoon. The glacial tongue is famed for its ice caves, with groups offering tours.