I just tried booking an AE itinerary involving some TK legs. The seats show up fine on the website but are not bookable.

I called in to make the reservation and the agent, after talking with the ticketing desk, came back to tell me that the TK system appears down and that they are not able to reserve seats at this time. This has been going on for the last three hours apparently.

The UA website also doesn't display any TK availability right now.

They don't know when this will be fixed. The agent suggested I try again in an hour or tomorrow morning.