A TEENAGE boy has been sexually assaulted by three men at Beulah Park.

The boy, 15, was walking west along on Beulah Rd, near Brand St, about 2am on Sunday, when he was approached by the trio, who had been travelling in a white van in the opposite direction.

Two of the men overpowered the teenager while a third sexually assaulted him on the footpath, police said.

The attackers were described as Caucasian males aged in their early 20s, tall, of thin build and wearing dark clothing. They were not known to the victim.

The van was described as an older model white tradesman-style van, which had a 4cm-wide black stripe halfway up each side and running horizontally along the full length of the van.

It was described as dusty - but in good condition - with only driver and passenger windows and doors, two headlights (as distinct to four, sometimes found on newer models), tubing and a ladder on the roof that did not extend past the back of the van, a rear step across the back and a rusty exhaust pipe that extended beyond the rear of the van.

Sexual Crime Investigation Branch Detective Superintendent Damian Powell said forensic evidence had been found at the scene.

Det-Supt Powell said it appeared the attack was isolated and not planned and that police had no information to suggest teenage boys were being targeted.

He said the victim had been staying with relatives.

"We are certainly concerned this is a serious sexual assault. The nature and the circumstances do cause police concern," he said.

"It's a traumatic experience for any victim of serious sexual assault and it's no different in this case."

Anyone with information to contact Crime Stoppers on 1800 333 000 or at www.sa.crimestoppers.com.au.