Tributes have started to pour in for world 100 metres breaststroke champion Alexander Dale Oen, who has died aged 26.

The Norwegian was at a training camp in Flagstaff, Arizona, where he was reported to have suffered a cardiac arrest.

Per Rune Eknes, the president of the Norwegian Swimming Federation, told "This is just awful, especially for the family and close friends, and also the sport of swimming."

Norwegian media report Dale Oen, the Olympic silver medallist, was found on a bathroom floor at 9pm local time on Monday. He was taken by ambulance to the medical center where, despite repeated attempts at resuscitation, he was pronounced dead.

The Norwegian team coach Petter Lovberg said: "We are all in shock, this is an out-of-body experience for the whole team over here.

"Our thoughts go out to his family who have lost Alexander too early. Words cannot describe the grief they must feel.

"For us here in Flagstaff it is important now that we stand together and take care of each other.

"It is an unreal situation and we have some tough days ahead."

Quadruple Olympic breaststroke champion Kosuke Kitajima tweeted: "In shock over the passing of a dear friend and great rival. RIP Alex."

South African Cameron van der Burgh added: "To my greatest rival. My greatest friend. My brother in breaststroke. May you rest in peace. One love."