The Government Accountability Office has said the contractors that NASA tapped to carry out human spaceflight missions to the International Space Station have made progress when it comes to their spacecraft development efforts but continue to face schedule delays under the Commercial Crew Program.

GAO said in a report published Wednesday that there are three risks that could further hold up efforts of Boeing and SpaceX to meet NASA’s certification requirements for their crew transportation platforms.

Those include the aggressive schedule plans set by the two companies; safety and programmatic risks; and challenges faced by program officials with regard to the completion of phased safety assessments and the need to verify whether the contractors meet NASA’s human spaceflight standards.

The report noted the uncertainty in final certification dates and schedule delays raise concerns on whether the U.S. can develop a capability to send astronauts to and from the ISS once its crew transportation contract for Russia’s Soyuz spacecraft expires in 2019.

Boeing and SpaceX won contracts worth approximately $6.8 billion combined in 2014 to build their respective CST-100 and Dragon crew capsules for the NASA program.

The report came nearly a year after GAO called on NASA to create a contingency plan to ensure continued access to the orbiting laboratory beyond 2018.