apache2 無効なコマンド "SSLEngine"

httpdを再起動すると、次のエラーが発生します。私は何を見逃していますか？

[root@localhost ~]# service httpd restart
Stopping httpd:                                            [  OK  ]
Starting httpd: Syntax error on line 22 of /etc/httpd/conf.d/sites.conf:
Invalid command 'SSLEngine', perhaps misspelled or defined by a module not included in the server configuration

以下を使用してmod_sslをインストールしました。yum install mod_ssl openssh

Package 1:mod_ssl-2.2.15-15.el6.centos.x86_64 already installed and latest version
Package openssh-5.3p1-70.el6_2.2.x86_64 already installed and latest version

私のsites.confは次のようになります

<VirtualHost *:80>
#    ServerName shop.itmanx.com
    ServerAdmin [email protected]

    DocumentRoot /var/www/html/magento
    <Directory /var/www/html>
        Options -Indexes
        AllowOverride All
    </Directory>

    ErrorLog logs/shop-error.log
    CustomLog logs/shop-access.log
</VirtualHost>

<VirtualHost *:443>
    ServerName secure.itmanx.com
    ServerAdmin [email protected]

    SSLEngine on
    SSLCertificateFile /etc/httpd/ssl/secure.itmanx.com/server.crt
    SSLCertificateKeyFile /etc/httpd/ssl/secure.itmanx.com/server.key
    SSLCertificateChainFile /etc/httpd/ssl/secure.itmanx.com/chain.crt

    DocumentRoot /var/www/html/magento
    <Directory /var/www/html>
        Options -Indexes
        AllowOverride All
    </Directory>

    ErrorLog logs/shop-ssl-error.log
    CustomLog logs/shop-ssl-access.log    
</VirtualHost>