Nginx reverse proxy ssl authentication to a service url

I would like to configure nginx reverse proxy for a http web application but would like to configure the authentication using a authentication service URL.

I could make the authentication work using a .htpasswd file containing user name and password but would like to have a service (instead of a file) to do the authentication.

Is there any way I can point nginx reverse proxy to a authentication service URL ?

Warm Regards,
Sourav