Problem with Unicode strings using docbook2pdf and docbook2html under Linux

Hello All
I am creating a book using XML DocBook. I use docbook2pdf and docbook2html tools under Linux (Fedora Core 6). When dealing with English text, everything is fine, but when I want to enter text in my own language (Persian), these two tools, complain about Unicode strings. My main XML file looks like this:

<?xml version="1.0" standalone="no" encoding="utf-8"?>
<!DOCTYPE book SYSTEM "/usr/share/sgml/docbook/xml-dtd-4.3-1.0-30.1/docbookx.dtd"[
<!ENTITY introduction SYSTEM "
introduction.xml">
<!ENTITY quickstart SYSTEM "quickstart.xml">
<!ENTITY products SYSTEM "products.xml">
<!ENTITY sales SYSTEM "sales.xml">
<!ENTITY serials SYSTEM "
serials.xml">
<!ENTITY licenses SYSTEM "licenses.xml">
<!ENTITY customers SYSTEM "customers.xml">
<!ENTITY manufacturers SYSTEM "manufacturers.xml">
<!ENTITY platforms SYSTEM "
platforms.xml">
<!ENTITY categories SYSTEM "categories.xml">
<!ENTITY options SYSTEM "options.xml">
<!ENTITY documentations SYSTEM "documentations.xml">

<!ENTITY points SYSTEM "points.xml">
]>
<book>
    <bookinfo>
        <title>Software Lock Manager</title>
        <author>
            <firstname>Ali</firstname><surname>Majdzadeh Kohbanani</surname>

            <affiliation>
                <address><email></email></address>
            </affiliation>

        </author>
    </bookinfo>
    &introduction;
    &quickstart;
    &products;
    &sales;
    &serials;
    &licenses;
    &customers;
    &manufacturers;

    &platforms;
    &categories;
    &options;
    &documentations;
    &points;
</book>

I can not figure out the problem. By the way, the errors were reported as follows:

jade:/opt/slm/doc/sgmlSources/manual.xml:19:12:E: non SGML character number 132
jade:/opt/slm/doc/sgmlSources/manual.xml:19:14:E: non SGML character number 140
jade:/opt/slm/doc/sgmlSources/manual.xml:19:17:E: non SGML character number 133

jade:/opt/slm/doc/sgmlSources/manual.xml:19:29:E: non SGML character number 135

Any ideas?

Best Regards
Ali Majdzadeh Kohbanani