How to pass systemProperties when invoking exec:java plugin in maven?

I want to use the exec:java plugin to invoke the main class from command line. I can pass arguments from the command line using -Dexec.args="arg0 arg1 arg2", I don't know how to pass system properties. I tried '-Dexec.systemProperties="key=value"` but with no effect.

pom.xml looks like this:

  <plugin>  
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>exec-maven-plugin</artifactId>
    <configuration>
      <mainClass>ibis.structure.Structure</mainClass>
    </configuration>  
  </plugin>