Android : .aar library not included into .arr output

I have a Gradle project, composed of two subprojects: UI and backend library. The library on it's own relies upon another .aar library.

Extract from library's build.gradle:

dependencies {
    compile(name: 'ActionBarSherlock', ext: 'aar')
}

Extract from UI's build.gradle:

dependencies {
    compile project(":my_library")
    compile fileTree(dir: 'libs', include: ['*.jar']) // all jars
}

When I try to build the project, build fails with following error:

> Could not resolve all dependencies for configuration ':UI:_debugCompile'.
   > Could not find :ActionBarSherlock:.
     Required by:
         Project:UI:unspecified > Project:my_library:unspecified

I checked the .aar output for my_library and it does not contain ActionBarSherlock.arr in libs folder. How do I tell Gradle to include .arr files into library output?