Update: Amazon has announced that its 4K service is now live for Prime Instant Video users in the UK too. As long as you're a Prime customer, you'll be able to enjoy UHD content at no extra charge.
However the app is only compatible with Sony Ultra HD smart TVs right now, with models from Samsung and LG to be added in 2015.
Original story below...
The service has gone live in the US, while a spokesperson told TechRadar: "Today's news is US-only. We don't comment on future plans, please stay tuned."
Amazon's Ultra HD streams are available through Instant Video app or on any compatible smart TVs. As you'd expect, the selection is limited for now, with BBC's Orphan Black joining Amazon's originals and a handful of titles from Sony.
But the best news is that customers won't be asked to pay anything on top of their current Prime fee should they want to enjoy some 4K.