You are seeking for webinf
element. See ant manual for more details.
The nested webinf element specifies a FileSet. All files included in this fileset will end up in the WEB-INF directory of the war file. If this fileset includes a file named web.xml, the file is ignored and you will get a warning.
Use following snippet that combines several use cases.
<war destfile="${build.dir}/${ant.project.name}_${version}.war" webxml="${web.dir}/WEB-INF/web.xml">
<webinf dir="${web.dir}/WEB-INF">
<exclude name="web.xml"/>
<exclude name="classes/**"/>
<exclude name="lib/**"/>
<exclude name="flex/services-config.xml"/>
</webinf>
<lib dir="${war.web-inf.dir}/lib"/>
<lib file="${build.dir}/*.jar"/>
<fileset dir="${web.dir}">
<include name="img/*"/>
</fileset>