A computer program that can edit videos of people speaking to realistically make it look like they said something else has been created, raising fears of clips being tampered with online.

Researchers at the University of Washington have lip-synced a video of former US president Barack Obama using the program to superimpose new audio onto the clip.

The realistic results put words in Obama's mouth by converting audio sounds into mouth movements and blending them onto an existing video of speech.

The effect could be used in special effects and to improve the quality of video calls, the researchers said.

"When you watch Skype or Google Hangouts, often the connection is stuttery and low-resolution and really unpleasant, but the audio is pretty good," said Steve Seitz, co-author of the research and professor at the University of Washington. "So if you could use the audio to produce much higher-quality video, that would be terrific."

Until now, video lip-syncing involved hours of filming and editing. But the computer program can create a clip with new audio after analysing one hour of speech rather than 14.