PHOTOS: American teenagers Ledecky, Franklin sizzle at Worlds

American teenager Katie Ledecky smashed the world record in the women's 1,500 metres freestyle final by more than six seconds during a glittering session for the United States at the world championships on Tuesday.

The 16-year-old from Bethesda, who also won the 400 freestyle gold medal on Sunday, powered away from defending champion Lotte Friis of Denmark in the final stages to break the previous best of 15:42.54 set by compatriot Kate Ziegler in 2007.

Ledecky sat on the shoulder of Friis for much of their two-horse race but the Dane was left trailing by a powerful late charge from the Olympic 800 metres champion and finished with silver in 15:38.88, also inside Ziegler's mark.

Lauren Boyle of New Zealand was third in 15:44.71 ahead of Spain's Mireia Belmonte Garcia in 15:58.83.

"It was really tough, my hardest race ever," Ledecky told reporters.

"I knew we were going pretty fast and I figured that whoever came out on top was probably going to get the world record.

"I had to be careful not to push it too early or push it too late and just touch the wall first. Around the last 200 I knew I could take off."

Succeeding Ziegler as the fastest woman over 1,500 metres was extra special, Ledecky said, as the 2011 world championship silver medallist behind Denmark's Lotte Friis hails from the same part of the United States.

"It means the world to me," said a still-dripping Ledecky, goggle marks visible around her eyes.

"Kate Ziegler is from my area and I have looked up to her my whole life.

"I am really honoured to break that world record and to keep it in the Potomac valley.

"I'm still 16 so I hope I have plenty years left of swimming and I guess we'll see where my limit is."

American Olympic champion Missy Franklin won her second gold of the world swimming championships with victory in the women's 100 metres backstroke.

The 18-year-old powered home in a time of 58.42 seconds at the Palau Sant Jordi following Sunday's 4x100 metres freestyle relay triumph.

Franklin had a gruelling day as she had to qualify for Wednesday's 200 metres freestyle final around an hour after claiming backstroke gold.

It was the 18-year-old's fifth world championship title, already putting her fourth among her compatriots.

"It was tough but it was awesome. It was nice to have the final before the semi-final - it was the other way around in London," she said.

American Matt Grevers won gold in the men's 100 metres backstroke.

Grevers, the Olympic champion, touched in a time of 52.93 seconds, with compatriot David Plummer taking silver in 53.12 and France's Jeremy Stravius bronze in 53.21.

Ryan Lochte, the second-most decorated men's world championship medallist after compatriot Michael Phelps, provided a rare moment of disappointment for the US team when he could manage only fourth in the 200 metres freestyle final.

Lochte, 28, who is swimming seven events this week, said he had made crucial mistakes.

"I am a bit disappointed with the time but you know I've got to keep moving forward," he said on Spanish television.

"There were a lot of places where I messed up but hopefully I can fix it in the next three years (before the 2016 Olympics) and I can make it better."

Frenchman Yannick Agnel followed up on his triumph at last year's London Olympics to take gold and Conor Dwyer salvaged some pride for the U.S. with silver.

"We've had an absolutely incredible evening," an excited Franklin, who is swimming eight events in Barcelona, said.

"This medal means a lot," Agnel, who has recently started training with Phelps's former coach Bob Bowman, told reporters.

"I just wanted to enjoy myself, I was relaxed," he added.

"Even if a world title is less important than an Olympic title this is hugely significant."