Invalid AuthExternal keyword (pwauth)

Turns out, the AddExternalAuth and SetExternalAuthMethod must be placed within the VirtualHost declaration. Hence the following config works just fine:

<VirtualHost *:80>
    ServerAdmin webmaster@localhost 
    DocumentRoot /var/www/html

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

    AddExternalAuth pwauth /usr/sbin/pwauth
    SetExternalAuthMethod pwauth pipe

    <Directory /var/www/html/sec>
        AuthType Basic
        AuthName "Restricted Area"
        AuthBasicProvider external
        AuthExternal pwauth 
        Require user pat
    </Directory>
</VirtualHost>

I still do not understand why the scenario in which the declaration is done globally, hence directly in apache2.conf, doesn't work. Nonetheless, the initial issue is solved....