The state attorney's office in Duisburg in North Rhine-Westphalia (NRW) state said on Thursday that three men in all had been detained in neighboring Dinslaken.

NRW Interior Minister Ralf Jäger had said Wednesday that the first of the four - a 24-year-old Syrian asylum seeker - had been arrested at Mutterstadt in adjacent Rhineland-Palatine state last Friday.

Court custody orders were being sought for all four.

His arrest had resulted from a "vague tip" from someone who thought he or she had "heard something," Jäger told reporters.

The tipoff related to a Bundesliga football match, although authorities could not provide precise details.

"So far, however, we have no concrete indications on a target or a location," said a spokeswoman at the public prosecutor's office in Duisburg, Anna Christiana Weiler on Thursday.

Threat unclear

Weiler said it was not clear whether the rumored attack plan was intended for a first or second division - Bundesliga or 2. Bundesliga - match.

Many of Germany's top clubs are based in the two western states. The 2. Bundesliga season began last weekend, with the top division's first matches of the new season scheduled for August 26, 27 and 28.

All four suspects had lived in a hostel for asylum seekers in Dinslaken.

Chief Duisburg prosecutor Detlef Nowotsch said the first arrest in Dinslaken had occurred on Wednesday morning. The other two were arrested later on Wednesday.

Germany on edge

Germany has been on edge since the so-called "Islamic State" militia, notorious for its brutal regime in parts of Syria and Iraq, claimed responsibility for two attacks in Bavaria last month.

Both assailants at Ansbach and Würzburg were killed and multiple people were wounded.

On Thursday, Federal Interior Minister Thomas de Maiziere presented a "public safety" plan, including boosts in staffing and equipment for police.

Opposition parties, including the Greens, said German law already provided sufficient scope to handle potential terrorist threats.

ipj/msh (AP, dpa, AFP)