A four-year-old child is missing in a house fire in Sydney's southwest.

Emergency services were called to the property at Duterreau Way, Claymore shortly before 2.30pm (AEST) on Tuesday where they found a townhouse alight.

The mother has told police that she believes the child is still in the house, a police spokeswoman told AAP.

A highway patrol officer attempted to enter the house, but was forced back by the flames.

He was treated on sight for smoke inhalation.

The blaze has gutted the building's top floor while the ground floor remains alight, Fire and Rescue NSW said.