rewritecond exclude url in .htaccess

[admin]

I have these working rewrite rules :

# Allow access to assets folder from plugins folders
RewriteRule ^app/plugins/(.+)/assets - [L]

# forbid access to files and folders under app
RewriteRule ^app/.*$ - [L,F]

# rewrite to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?action=$1 [L,QSA]

Now I would like to exclude from these rules everything located under <a href="http://www.mywebsite.com/wordpress/" rel="nofollow">http://www.mywebsite.com/wordpress/</a>...
Therefore I've added a new rewrite condition :

...
# rewrite to index.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^\/wordpress\/ [NC]
RewriteRule ^(.*)$ index.php?action=$1 [L,QSA]

But it dosen't work. When I try to access something like this :
<a href="http://www.mywebsite.com/wordpress/contact-us/" rel="nofollow">http://www.mywebsite.com/wordpress/contact-us/</a>

it redirects me to index.php...

Any help would be really appreciated.