“There are still smouldering fires so the building is difficult to access,” Andreas Leandersson, emergency services spokesman at the southern Älvsborg rescue service, told Swedish newspaper, Aftonbladet.
In a separate interview, Göran Carlbom, duty officer for the region's police force, told the TT newswire that it is too early to say if the fire was started deliberately.
“At present there are no such suspicions. We must talk to the patrol and rescue services first,” Carlbom said.
The extent of the damage that the building sustained was difficult to determine on Sunday morning but police confirmed that arson is one arm of their investigation.
“We have a patrol on the site and will start the investigation as soon as possible,” a police spokesman told TT.
"It is no longer possible to use the site. All activities have been cancelled until further notice," the mosque's administration said on Facebook.
Although arson aimed at mosques is relatively rare, Sweden has suffered a rash of attacks on refugee housing over the last year, with investigations launched into more than two dozen suspected arson attacks, mostly linked to the growing influx of migrants.
Mona Sahlin, the national coordinator for protecting democracy against violent extremism, has been critical of “inflammatory” rhetoric being spread in the public debate and on social networks.
“I'm certain that it is part of the far-right extremist movement to use the refugee situation to strengthen their arguments... It was the same 20 years ago,” Sahlin told the AFP news agency in November.
In 1992, 52 attacks were carried out against centres housing refugees from the Balkans war.
Sweden recently introduced border checks after running out of accommodation for asylum seekers, having recently taken in more per capita than any other EU country.
Almost 163,000 refugees sought asylum in Sweden in 2015.