A one-year-old child was among 13 people who have been rescued from a fire at a block of flats.
The blaze broke out at a property in the Carnhill area of Londonderry just before 6.30am.
Northern Ireland Fire and Rescue Service (NIFRS) area commander Trevor Ferguson said: "The early indications are that this fire was started deliberately."
A number of carpets in the communal hallway of the two-storey building are believed to have been set alight.
Mr Ferguson said the local firefighters were on the scene within minutes.
He added: "Fire crews were faced with a very difficult and potentially very tragic set of circumstances. There was a developing fire in the communal stairwell."
They used ladders to rescue a family including the one-year-old child and another man from flats on the first floor.
Five others were rescued using the internal stairwell by crews wearing breathing apparatus.
A further four occupants managed to escape the burning building before the fire service arrived.
Mr Ferguson has praised the actions of his team, which he said saved lives.
"This could have had potentially very tragic consequences. If it had not been for the prompt and professional actions of fire crews we could have been looking at a much more devastating outcome," he said.
The fire was extinguished after about an hour but Police Service of Northern Ireland (PSNI) and NIFRS investigators are still at the scene.
A spokeswoman for the PSNI appealed for anyone with information to contact officers at Strand Road in the city.
SDLP MLA Colum Eastwood praised the quick response of the fire service.
Mr Eastwood said: "This must have been absolutely awful for the residents, especially as it happened early in the morning. It must have been extremely traumatic. The NIFRS deserve great praise for saving lives today.
"I just can't understand the mindset of those behind what is a suspected arson attack and I would appeal to anyone with information to contact the police immediately."