What HTTP status code should I use for a REST service?

I'm designing a REST service. This REST service can be invoked by POST and in some cases to continue the processing an action by the user is needed.

I don't know what HTTP status code I've to use to communicate it.

It can be 200 with a return code in the body responde, but it isn't really OK.

It can be 300, but it isn't a redirect.

What do I use?

Thanks a lot!