Disabling hyphenation on selected string

Hello,
I've tried to disable hyphenation on a selected string inside a 
paragraph where hyphenation is enabled.
This is my document.xml sample:

<para>
    This sentence can be hyphenated anywhere.
    <phrase role="nohyph">On the opposite this sentence shouldn't be 
hyphenated.</phrase>
    This sentence can be hyphenated anywhere again.
</para>

Moreover in my stylesheet for PDF output (mystyle.xsl) I've defined the 
following attribute-set and template

    <xsl:attribute-set name="nohyph.prop">
        <xsl:attribute 
name="keep-together.within-line">always</xsl:attribute>
        <xsl:attribute name="hyphenate">false</xsl:attribute>
    </xsl:attribute-set>

    <xsl:template match="phrase[@role='nohyph']">
        <fo:inline xsl:use-attribute-sets="nohyph.prop">
            <xsl:apply-templates/>
        </fo:inline>
    </xsl:template>

However this trick doesn't works because I see that the sentence in 
phrase "nohyph" is hyphenated.

Has anyone any suggestions about this issue?

Thanks and bye,

Demis Biscaro