Is Foreign travel to the Donbass allowed?

Has anyone travelled to Donetsk, Lugansk or Mariupol?

There seem to be a few hotels that are open in these cities according to Yandex maps. And there is bus service from the Russian side.

I'm thinking of taking a bus from Rostov On Don. Has anyone tried this? I'm curious if a USA passport and russian tourist visa will be enough.

I've been there a couple of times. You need an invitation from someone there.

I've been chatting with a hotel in Mariupol, and they say foreign tourists have not needed any invitation letter.