Xml – [taskdef] Could not load definitions from resource testngtasks

Selenium Webdriver+testNG+ANT
I have an error running -ant runtest
C:\seleniumtests>ant runtest
Buildfile: C:\seleniumtests\build.xml

compileTests:
[jar] Building jar: C:\seleniumtests\AntruTests.jar

runtest:
[taskdef] Could not load definitions from resource testngtasks. It could not b
e found.

BUILD FAILED
C:\seleniumtests\build.xml:38: Problem: failed to create task or type testng
Cause: The name is undefined.
Action: Check the spelling.
Action: Check that any custom tasks/types have been declared.
Action: Check that any / declarations have taken place.

Total time: 1 second

build.xml:

    <?xml version="1.0" encoding="iso-8859-1"?>

<project name="AntruTests">
    <!-- Properties storage -->
    <property file="build.properties"/>

    <!-- Project's folders locations -->

    <property name="project.path" value="." />

    <!-- Set class path libraries to be used for compilation -->
    <path id="class.path">
    <pathelement location="lib" path="lib/selenium-java-2.37.0.jar"/>
    <pathelement location="lib" path="lib/selenium-server-standalone-2.37.0.jar"/>
    </path>


     <!-- Title for ReportNG -->
    <property name="report.title" value="Automated tests report for AntruTests"/>

    <!-- Compile classes -->
    <target name="compileTests"> 
        <javac classpathref="class.path"  includeantruntime="false" destdir="C:\seleniumtests" encoding="UTF-8" optimize="off" 
            debug="on" failonerror="true" srcdir="C:\seleniumtests" />

            <jar destfile="AntruTests.jar" basedir="C:\seleniumtests" />
        </target>

    <target name="prepareForRunning" depends="compileTests">

        <delete dir="${tests.results.folder}" />
        <mkdir dir="${tests.results.folder}" />
</target>


    <target name="runtest" depends="compileTests" description="Runtests">
        <taskdef resource="testngtasks" classpath="${lib.dir}/testng-6.8.7.jar"/>
<testng outputdir="${testng.output.dir}" classpathref="classes">  
<xmlfileset dir="${lib.dir}" includes="testng.xml"/>  
             </testng>

</target>
</project>

What I am doing wrong?
Thanks!