A 27-year-old Syrian man denied asylum in Germany a year ago blew himself up on Sunday outside a crowded music festival, injuring 12 people in the country's fourth violent attack on members of the public in less than a week.

A spokeswoman for the Bavarian state police force said on Monday it was unclear whether the man was an Islamist militant, and that investigations were continuing.

German newspaper Die Welt quoted Bavarian Interior Minister Joachim Herrmann earlier as saying: "My personal view is that it is unfortunately very likely that a real Islamist suicide attack took place here."

The incident, on top of three other attacks since July 18 that left 10 people dead and 34 injured, will fuel growing public unease about Chancellor Angela Merkel's open-door refugee policy. More than a million migrants have entered Germany over the past year, many fleeing war in Afghanistan, Syria and Iraq.

Police said three of the 12 wounded were in a serious condition after the attack in Ansbach, a town of 40,000 people southwest of Nuremberg that has a US Army base.

The dead man had been in treatment after twice before trying to kill himself, though Sunday evening's explosion was more than just "a pure suicide attempt", Herrmann said. An Islamist link could not be ruled out, he said earlier.

"It's terrible that someone who came into our country to seek shelter has now committed such a heinous act and injured a large number of people," Herrmann said early on Monday.

"We must do everything possible to prevent the spread of such violence in our country by people who came here to ask for asylum."

Motive still unknown

Herrmann said the Syrian asylum seeker arrived in Germany two years ago and had been in trouble with local police repeatedly for drug-taking and other offenses.

He said investigators had yet to determine the motive of the attacks. "Because the rucksack and this bomb were packed with so many metal parts that could have killed and injured many more people, it cannot simply be considered a pure suicide attempt."

It was the second violent incident in Germany on Sunday and the fourth in the past week, including the killing of nine people by a deranged 18-year-old Iranian-German gunman in the Bavarian capital Munich on Friday.

Herrmann said the man, whose identity has not yet been released, had been living in Ansbach for a year. Although his application for asylum had been denied, he was not in danger of being deported immediately given the civil war in Syria.

One US intelligence official said investigators would focus on what the bomber was doing before he left Syria and why he was denied asylum.

US security sources said the bombing did not appear to be a well-planned operation and could well turn out to be the act of another mentally unstable individual.

Herrmann said the man had been denied entry to the Ansbach Open music festival shortly before detonating the bomb outside a restaurant. More than 2,000 people were evacuated after the explosion.

Earlier on Sunday, a 21-year-old Syrian refugee was arrested after killing a pregnant woman and wounding two people with a machete in the southwestern city of Reutlingen, near Stuttgart.

Recent major attacks in Germany

A string of bloody attacks have rocked Germany of late, setting the nerves of its public on edge and raising concerns about safety and refugee issues. The following is a summary of the attacks in Germany over recent months.

On Dec 31, a group of some 1,000 men surrounded, harassed and robbed mostly women in the western German city of Cologne. According to eyewitnesses and victims, the perpetrators were from Arabic or North African regions.

On May 10, a man stabbed four people at a train station in a random early-morning attack in Grafing near Munich. One man later died.

On June 23, a masked gunman stormed a cinema in western Germany and took some people hostage. He was shot dead by police. No one was injured.

On July 18, a 17-year-old asylum-seeker from Afghanistan assaulted passengers with axe and knife on a regional train near Wuerzburg, Bavaria, severely wounding five. The attacker was shot dead by police.

On July 22, an 18-year-old with dual German-Iranian nationality killed 10, including himself, and wounded 27 others at a shopping center in Munich.

On July 24, one person was killed and 12 others were injured in an explosion in the southern German city of Ansbach. The dead was believed to be the attacker.

On the same day, a man killed a woman and injured two others with a machete in the southwestern German town of Reutlingen. Authorities identified the attacker as a 21-year-old asylum seeker from Syria.