According to an urgent message from NASA, an "anomaly" was detected in the primary booster and a decision was taken to abort the launch. This incident occurred while the rocket was in flight, thus requiring a ballisting landing by the crew.
This incident occurred around four hours ago (at 7pm AEST)
The full NASA statement is as follows:
“The Soyuz MS-10 spacecraft launched from the Baikonur Cosmodrome in Kazakhstan to the International Space Station at 4:40 a.m. EDT Thursday, Oct. 11 (2:40 p.m. in Baikonur) carrying American astronaut Nick Hague and Russian cosmonaut Alexey Ovchinin. Shortly after launch, there was an anomaly with the booster and the launch ascent was aborted, resulting in a ballistic landing of the spacecraft.
“NASA Administrator Jim Bridenstine and the NASA team are monitoring the situation carefully. NASA is working closely with Roscosmos to ensure the safe return of the crew. Safety of the crew is the utmost priority for NASA. A thorough investigation into the cause of the incident will be conducted.”
More information as it becomes available.
Update (11:30pm AEDT) NASA TV coverage is available here.