Nordic countries will increase police cooperation and form a hub in Stockholm in an effort to prevent Sweden's serious gang crime problem from spreading to Norway, Finland and Denmark, the Swedish government said on Wednesday.

Swedish gangs have long been among the most violent in Europe but this year 10 Swedes - over half of them minors - have been charged in Denmark with attempted murder or weapons possession, stirring alarm and harsh criticism in Denmark.

Swedish Justice Minister Gunnar Strommer told Reuters that in response, Sweden and Denmark would post police officers in each other's countries.

"An important reason for this is to be able to share information between our countries in real time," Strommer said. "Both to be able to identify at an earlier stage which children and young people are at risk of being drawn into these criminal networks, and to prevent it."

In addition, a Nordic hub of police officers from Finland, Norway and Denmark would be set up in Stockholm. Norwegian and Finnish officers are already there, and the Danes would join in a few weeks' time.

Swedish gang members have been hired by Danish gangs for violent attacks on rivals and Danish Justice Minister Peter Hummelgaard said there were 25 such instances just since April.

He labelled them "child soldiers" and said Sweden had a "sick" culture of violence not wanted in Denmark.