Residents return home after Sydney fire

More than 20 residents forced to flee their unit block after a fire broke out in an abandoned neighbouring building in Sydney's south have been allowed to return home.

Flames engulfed the former RSL club on Botany Road at Mascot at 1am on Tuesday, forcing 21 people in the block next door to be evacuated.

A NSW Fire & Rescue spokesman said crews concentrated on saving the surrounding unit blocks.

"The building was due for demolition," he said.

There are no reports of injuries.

Fire and forensic investigators are at the scene in an attempt to establish what caused the blaze.

Traffic remains slow around the area with one of two southbound lanes of Botany Road closed.