How to see the iptables logs in systemd

I have a system with Debian Jessie. It comes with systemd. I want to see the kernel logs for iptables.

I have a bunch of iptables and at the end I want to log what doesn't get allowed. I do it like this:

iptables -A INPUT -j LOG

With syslog I could see the iptables logs doing this:
tail -f /var/log/syslog | grep kernel
With systemd I can't find a way to see what is failing. I've been trying journalctl and journalctl -f.
journalctl alone shows a lot, doing a grep -v of what I am not interested I still can't see anything about iptables.

I also tried what is supposed to show kernel messages but I see nothing but when it started logging:

journalctl -k

I am trying to find out if iptables is a service or what, but I can't see it with:

systemctl --full list-units --type=service

I tried following this post about systemd and logs in general, but I still get no clue about how can I see iptables logs: https://www.digitalocean.com/community/tutorials/how-to-use-journalctl-to-view-and-manipulate-systemd-logs

Any hints ? thank you very much.