Mohsen Fakhrizadeh. (Source: peoplesdispatch.org)

Germany also called on all sides to show restraint and avoid escalating tensions that could derail any talks on Iran's nuclear program.

It is important to preserve the scope for talks with Iran so that the dispute over Iran's nuclear program can be resolved through negotiations, a German foreign ministry spokesman said.  

Syria’s Foreign Ministry called the killing a “terrorist act” which could fuel tensions in the region.

Qatar’s Foreign Ministry condemned the assassination and called for self-restraint. 

Mohsen Fakhrizadeh died after armed assassins fired on his car on Friday, Iranian media reported.

Adviser to Iranian Supreme Leader Ayatollah Ali Khamenei said Iran would strike back against the killers.

No group has claimed responsibility for the attack.