Four fire engines and around 25 firefighters were called to a fire at a flat in Belvedere Road in Waterloo.
Almost half of a four-roomed flat on the fifth floor of a 12 storey-block was damaged by the fire. Around 20 people evacuated the building and a number of other people remained in their unaffected flats. Three men were treated at the scene for smoke inhalation by London Ambulance Service crews.
Watch Manager Glynn Williams, who was at the scene, said: "Crews worked hard to locate and bring the fire under control very quickly.
"Sadly, two cats were found dead in the flat.
"This incident shows the importance of a smoke alarms as the flat was fitted with a smoke alarm which alerted the residents to the fire.
"It is important to have working smoke alarms fitted on every level of your home.
“Smoke alarms give the earliest possible warning when a fire starts and we would urge everyone to make sure they have one fitted in every room where a fire can start except kitchens or bathrooms where heat alarms are more appropriate.”
The Brigade was called at 2207 and the fire was under control at 2323. Fire crews from Lambeth, Dowgate and Soho fire stations attended the scene.
The cause of the fire is under investigation.