Katie Ledecky reacts after breaking the world record of the women's 1,500-meter freestyle at the FINA World Aquatics Championships in Kazan on Monday. Photo: AFP

American teenager Katie Ledecky bettered her own 1,500-meter freestyle world record with a superb display during the morning heats at the world championships on Monday.

The 18-year-old, who won women's 400-meter freestyle gold on Sunday, recorded 15 minutes 27.71 seconds to lower her previous mark, which was set at the Pan Pacific Championships in August 2014, by 0.65 seconds.

Ledecky was inside world ­record pace throughout her ­preliminary heat as the ­American finished 28.81 ­seconds ahead of her nearest competitor, Jessica Ashwood, the Australian who qualified third fastest for Monday's final.

It was the fourth time that Ledecky has broken her own 1,500-meter freestyle record. She first set the mark at the 2013 world championships, smashing compatriot Kate Ziegler's previous time from 2007 - 15:42.54 - by over 6 ­seconds.

"My coach told me to swim the first 900 meters easy, build over the next 300, then the final 300 was my choice and to be honest, it felt pretty easy," said Ledecky, who came within a second of her 400-meter record on Sunday.

"I wasn't kicking that much and breaking that record is ­testament to the work I have put in and the shape I am in right now.

"I was barely even focusing on this morning's swim as I was so relaxed. I realized just towards the end as I could see people waving and I knew where my parents, brother and uncle were sitting.

"I am pretty shocked that I was able to do that."

Ledecky only recently ­finished high school in ­Maryland, but this is the eighth time the teenager has set a world record in her fledgling career.

It is the fourth time the ­defending champion has ­broken the 1,500-meter record alone having first achieved the feat in Barcelona two years ago on her way to winning the world title.

This is the second world ­record to fall in Kazan after Sweden's Sarah Sjostrom feat in the women's 100-meter ­butterfly heats on Sunday.