Swedish Prime Minister Ulf Kristersson on Monday admitted he was concerned about "very serious" reports of police employees leaking information to gang members with whom they had been romantically involved.

Sweden has struggled to contain a surge in violence in recent years as criminal gangs feud for control of drug markets, with bombings and shootings recorded weekly.

Kristersson's comments followed a report by the newspaper Dagens Nyheter (DN) that claimed to have found multiple instances of police employees leaking sensitive information about ongoing investigations to criminals.

In some cases, the police employees allegedly were in intimate relationships with gang members to whom they leaked information. According to DN, which published the report over the weekend, in four cases the leaks -- which included information about enemies of the gang member involved in the relationship -- preceded revenge attacks, including murders.

At least 30 employees had for different reasons been considered "security risks" and either resigned or were forced to quit, the newspaper reported. DN said that in several cases, criminals had begun "sexual relations with strategically selected police officers".

Kristersson on Monday told news agency TT that it was "very concerning information". "There are many great risks and one is that trust in police declines, that one gets the idea that mafia-like methods are used to infiltrate law enforcement," the head of government said.

"It is very serious and we need to address it," he continued. Kristersson said that purely based on the initial report he could not say whether it constituted a threat to national security or not. "But the mere suspicion of these types of connections are damaging," he told the news agency.