A Syrian migrant who blew himself up outside a music festival venue in southern Germany pledged allegiance to the Islamic State extremist group in a video found on his phone, Bavaria's top security official said in Berlin today.

The 27-year-old Syrian, whose asylum application was turned down in Germany, vowed to take "revenge against the Germans for obstructing Islam," Bavarian Interior Minister Joachim Hermann told a press conference.

The man from Aleppo, in northern Syria, injured 15 people when he detonated an explosive device in the Bavarian town of Ansbach about 10pm local time on Sunday night, after being denied entry to the event because he did not have a ticket.

Federal prosecutors said later yesterday that the suspect was a member of Islamic State.

The federal prosecutors' office will take over the investigation because it appears to be connected to a foreign terrorist organisation, it said in Karlsruhe.

Islamic State, via its Aamaq News Agency mouthpiece's channels on the Telegram messaging app, said the bomber was one of its "soldiers" and was responding to its call to target members of an international coalition fighting it.

Aamaq later published what it said was a video of the attacker. It showed a man with his face masked saying he renewed his pledge of allegiance to Islamic State leader Abu Bakr al-Baghdadi.

The speaker, who had a Syrian accent, said he would carry out a "martyrdom operation" with a bomb in Ansbach "in response to the crimes being committed by the coalition, with Germany, in bombing men, women and children."

German police raided the man's refugee hostel and confiscated bomb-making items - including fuel, hydrochloric acid and alcohol-based cleaner - as well as material related to Salafism, an ultra-conservative reform movement within Sunni Islam.

A suspected accomplice, who claimed to have known the attacker only in his capacity as a translator, was arrested early today.

The Syrian man, who detonated a rucksack laden with explosives and shrapnel in central Ansbach at about 10pm local time on Sunday (4am yesterday in Malaysia), came to Germany in August 2014.

He was denied asylum several months later and had been due for deportation to Bulgaria, the first country where he registered for asylum.

Interior Minister Thomas de Maiziere said the deportation to Bulgaria had been suspended because of his "psychological instability."

The so-called Dublin principle observed by many countries determines that asylum seekers who have passed through a safe third country where they could have claimed asylum can be sent back there in order to make their claim.

The man had been known to police for drug-related and other crimes, investigators said. He had received treatment at a psychiatric clinic and had tried to kill himself on two prior occasions.

'Bomber known to be a compulsive liar'

Alireza Khodadadi, a man who shared the bomber's refugee accommodation, told the N24 broadcaster that the Syrian had told him he did not identify with Islamic State ideology. The witness added that the bomber had been known among peers as a compulsive liar.

The Ansbach bombing was the fourth violent attack to rock Germany in the past week. Three of them were perpetrated by recent immigrants, reigniting debate over how Germany should deal with the 1.1 million migrants who arrived in the country in 2015.

Last week, a 17-year-old registered in Bavaria as an Afghan asylum seeker attacked passengers on a commuter train with an axe and a knife, injuring five people. Islamic State claimed responsibility for that attack.

On Sunday, a 21-year-old Syrian asylum seeker killed a woman and injured four others with a deli knife in Reutlingen, in what investigators say was not an act of terrorism.

German Chancellor Angela Merkel is "mourning with the families of the woman killed in Reutlingen" and her "thoughts are with the families of those injured in Reutlingen and Ansbach," her deputy spokeswoman Ulrike Demmer said Monday.

The European Union said its compassion for refugees will not come at the expense of security in the bloc.

"Europe's doors are open to those who flee war and persecution and who seek asylum, but we will defend ourselves against attacks on our way of life," European Commission spokesman Margaritis Schinas said in Brussels.

"Our sense of compassion does not and must not come at the expense of our security," he added.

- dpa