Russian news agencies reported that the crew had safely made an emergency landing and were in radio contact and that rescuers were en route to pick them up.
Nasa says a booster failure forced the two crew to return to return to Earth in a "ballistic descent mode" - a sharper angle of landing compared to normal.
The Kremlin confirmed the men had survived.
Roscosmos and NASA said the three-stage Soyuz booster suffered an emergency shutdown of its second stage.
A Soyuz rocket takes off from Baikonur Cosmodrome.
The rocket was carrying USA astronaut Nick Hague and Russian cosmonaut Alexei Ovchinin. Soyuz is now the only means for crews to travel to and from the station, with commercial crew vehicles by Boeing and SpaceX not expected to be ready to enter service before the middle of 2019. Search and rescue crews are heading to the landing site.
It was the first space mission for Hague, who joined NASA's astronaut corps in 2013. Hague and Ovchinin will spend about six months living and working aboard the orbiting lab.