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A Syrian with suspected links to the Islamic State group who blew himself up outside a German music festival was in contact with another person "who influenced the attack", authorities alleged on Wednesday.

The 27-year-old failed asylum-seeker, who wounded 15 people at a nearby cafe late Sunday when he was refused access to the festival venue, had been speaking to an unknown person in an "intensive" online chat, Bavaria state interior minister Joachim Herrmann said.

"Apparently he had direct contact with someone who had significant influence on the way the attack played out," Herrmann was quoted by DPA news agency as saying on the sidelines of a state government meeting.

"The chat ended immediately before the attack."

Herrmann said it was not immediately clear whether the unknown person had contact with IS extremists, where the chat participant was, or how long the two had been in contact.

He added that it was unclear whether the assailant meant to detonate the bomb at the moment he did.

"Because of witness testimony on what happened and also the course of the chat, there are indeed questions about whether he intended to set off the bomb at that moment," he explained.

On Monday, Herrmann revealed that the attacker had made a video pledging allegiance to IS leader Abu Bakr al-Baghdadi that was found on his smartphone.

IS later claimed through the Amaq news agency that the attacker "was a soldier of the Islamic State" who had acted "in response to calls to target nations in the coalition fighting" the extremists.

The assailant, who came to Germany two-years-ago but had his asylum claim rejected after a year, had tried to kill himself twice in the past and had spent time in a psychiatric clinic, authorities said.

Because of witness testimony on what happened and also the course of the chat, there are indeed questions about whether he intended to set off the bomb at that moment. - Joachim Herrmann

According to the IS online weekly magazine al-Nabaa, the assailant spent months planning the attack, once even hiding his home-made bomb in his room moments before a police raid.

The report, published late on Tuesday, added that he had fought both in Iraq and Syria with a branch of al-Qaeda and IS before arriving in Germany as an asylum-seeker two years ago.

He also helped the group with its propaganda efforts, setting up pro-IS accounts online.

In Germany, he started making the bomb, a process that took him three months, al-Nabaa alleged.

Germany was already reeling after nine people were killed in a shopping centre shooting spree in Munich on Friday and four passengers on a train and a passer-by were wounded in an axe attack in Wuerzburg on 18 July.

IS also claimed the axe rampage.

All three brutal incidents were in Bavaria, the southern state which has been a portal for tens of thousands of refugees under Chancellor Angela Merkel's liberal asylum policy.

Conservative lawmakers have called for an increased police presence, better surveillance and background checks of migrants, and new strategies to deport criminal asylum seekers more easily.