dependencySets not allowed within binaries tag

On http://maven.apache.org/plugins/maven-assembly-plugin/assembly.html we've got
<assembly>
...
<moduleSets>
<moduleSet>
....
<binaries>
<dependencySets>
<dependencySet>
....

But i've got an error if i do that :

[ERROR] BUILD ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Error reading descriptor

Embedded error: Unrecognised tag: 'dependencySets' (position: START_TAG seen ...
</unpack>\r\n <dependencySets>... @18:33)

I've got no way to specify scope of the dependencies i want to include because

...
<includeDependencies>false</includeDependencies>
<dependencySets>
<dependencySet>
<scope>system</scope>
</dependencySet>
</dependencySets>
...

Does not work