"We understand that this incident may cause concern for the local community and I would like to reassure residents that there is no risk to members of the public.
"The incident has happened inside a property and those involved are believed to have been known to each other.
"Local officers are out and about in the community to offer reassurance and aid the investigation."
More top news
Hundreds have left the Civic Centre in Sunderland as this year's Sunderland Pride gets underway.
It happened after yesterday's football game, which saw Barrow win 2-0 against York City.
All of the fires were extinguished but police are appealing for information. They believe they may have been started 'maliciously'.