Three Syrian men have been arrested in Germany over an alleged plan to launch a suicide attack there for the Islamic State of Iraq and the Levant, federal prosecutors said Thursday.

According to the suspects' plan, two attackers were to detonate suicide vests while the others were to kill passersby with guns and explosives in the western city of Dusseldorf, prosecutors said in a statement.

The suspects were identified as 27-year-old Hamza C., 25-year-old Mahood B., and Abd Arahman A. K., 31, who were arrested in the states of Brandenburg near Berlin, North Rhine-Westphalia and Baden-Wurttemberg.

A fourth Syrian man, Saleh A., 25, had been in custody in France since turning himself in in February, they said, and Germany had now requested his extradition.

"According to current investigations, the four accused were planning to commit an attack in Germany for the foreign terrorist organisation Islamic State," said the prosecutors.

Saleh A. and Hamza C. had joined Isil in early 2014 in Syria, where "they were ordered by the group's leadership to carry out an attack in the old town of Dusseldorf".

"Two suicide attackers were to set off their explosives vests at the Heinrich-Heine-Allee, a major street in Dusseldorf," the prosecutors said in the statement.