Germany warns against anti-refugee suspicions amid attacks

"We must not place refugees under general suspicion despite individual cases that are under investigation," he said following assaults in southern Germany, some involving asylum-seekers.

The sentiment was echoed by Chancellor Angela Merkel's deputy spokeswoman Ulrike Demmer who the government's "shock" at the violence.

 "Most of the terrorists who carried out attacks in recent months in Europe were not refugees," she said.

"This fact corresponds with ongoing investigations indicating that the terrorism threat (among refugees) is not larger or smaller than in the population at large."

On Sunday, a Syrian migrant set off an explosive device near an open-air music festival in southern Germany that killed himself and wounded a dozen others.

Germany is reeling after two other attacks in recent weeks.

Nine people died in a shopping centre shooting rampage in Munich on Friday and four people were wounded in an axe attack on a train in Wuerzburg on July 18.

Police said the man intended to target the music festival on Sunday but was turned away because he did not have a ticket, and set off the explosive device outside a nearby cafe.

The perpetrator was killed in the blast, police said in a statement, and a spokeswoman said 12 people were wounded, three of them seriously.

"Unfortunately, this is a terrible new attack which will surely increase people's anxiety," regional interior minister Joachim Herrmann said.

Herrmann said he was worried "the right to asylum would be undermined" by the events of the past week.

Europe has been on edge for months after a string of deadly attacks claimed by the Islamic State militant group, including bombings in Brussels and carnage at Bastille Day celebrations in the southern French city of Nice.