OSLO (Reuters) - Norwegian swimming world champion Alexander Dale Oen has died of a cardiac arrest in Arizona at the age of 26, the Norwegian Olympic Committee said on Tuesday.

Dale Oen, who took an emotional men's 100 meters breaststroke gold medal at last year's world championships in Shanghai, had been attending an altitude training camp.

A silver medalist at the 2008 Beijing Olympics, he was one of Norway's leading medal hopes for the London Games starting on July 27.

"He died suddenly, without warning on Monday evening local time while with the swimming team in Flagstaff, USA," the Olympic Committee and Norwegian Swimming Federation said in a statement.

Ola Ronsen, doctor to Norway's elite Olympians, was quickly on the scene and an ambulance arrived shortly afterwards but Dale Oen was pronounced dead at 2100 local time.

"My thoughts go first and foremost to his family in Oygarden," Per Rune Eknes, president of the Norwegian Swimming Federation said in a statement.

"This is the toughest day the sport of swimming in Norway has ever had."

He had struggled with his emotions after attacks that sent Norway into national mourning, touching the Norwegian flag on his swimming cap every time he entered the pool deck.

(Reporting by Alan Baldwin in London and Philip O'Connor in Stockholm, editing by Ed Osmond)