Reigning world and Olympic 800m champion David Rudisha has pulled out of next month's World Championships in Moscow because of a knee injury.

The 24-year-old Kenyan set a new 800m world record of one minute 40.91 seconds at last summer's Olympic Games in London.

"I regret I will not be able to defend my 800m title at the World Championships," he said.

"My knee problem is progressing well but unfortunately not fast enough to enable me recover in time."