INDIANAPOLIS (REUTERS) - A 6-year-old boy remained hospitalised in critical condition on Monday after he was buried for three hours in a sand dune at a popular Great Lakes beach area in Indiana.

Nathan Woessner sank 3.3m into the dune last Friday afternoon while on an outing with his family, US National Parks spokesman Bruce Rowe said.

Woessner is sedated and on a respirator as medical personnel try to remove sand from his lungs and run other tests, said Dr Tracy Koogler, medical director of Pediatric Intensive Care at the University of Chicago Medical Center.

He was improving faster than expected, she said.

It was unclear what caused the accident, Mr Rowe said. The location where it happened, Mount Baldy, has been closed indefinitely.

Rowe called the incident "unprecedented" in the history of the National Park Service. Indiana Dunes National Lakeshore, along Lake Michigan south of Chicago, is maintained by the National Park Service.

Mr Rowe said one possibility is that the remains of a tree at the bottom of the 126-foot (38-meter) dune created a void under the sand. Rescuers saw what may be traces of a rotting tree where Woessner was found, he said.