Los Angeles - Caeleb Dressel and Katie Ledecky will spearhead a powerful US assault when the FINA World Championships begin on Sunday, with the Americans looking to make a big statement before next year's Tokyo Olympics.

The event in Gwangju is the biggest meeting on the 2019 calendar, but for team USA it represents a stepping stone on the road to Tokyo - and one they expect to negotiate in characteristically confident fashion.

"While we understand that the worlds is an important competition, the most important competition for us is the Olympic Games," said national team managing director Lindsay Mintenko.

There have been no cut-throat trials to produce the US squad, which was selected based on performances in the 2018 national championships and 2018 Pan Pacific Championships.

That allows top swimmers to keep their training geared toward Tokyo and, as a result, few Americans feature at the top of the 2019 rankings.

Those that do include superstar Ledecky, who boasts leading times this year in the 400m, 800m and 1 500m freestyle and Lilly King, who is fastest in the 50m and 100m breaststroke as she prepares to renew her rivalry with Russian Yuliya Efimova.

Dressel, who two years ago in Budapest joined Michael Phelps as the only swimmer to win seven titles at a single world championships, tops the 100m butterfly rankings at 50.36 - faster than Singapore's Joseph Schooling clocked to win gold at the Rio Olympics.

Dressel's performance in Mission Viejo, California, last month fired a warning to rivals that by the time they complete their training camp in Shanghai the Americans will be a force to be reckoned with in South Korea.

"It's just really showing where we're at," said the 22-year-old, whose seven golds in Budapest included individual titles in the 50m and 100m freestyle and 100m butterfly along with four relay titles.

"I think I'm just in kind of a weird spot right now. I have to stay focused. I've got to get better. We'll go home, make adjustments, so we'll be ready."

Five-time Olympic champion Ledecky's gold medal haul from the last worlds in Hungary included the 400m, 800m and 1 500m freestyle and two relays.

Other returning champions from 2017 - when the US topped the medals table with 18 golds and 38 in total - include Chase Kalisz, who won both men's medleys in Budapest, and Simone Manuel, who stormed to the women's 100m freestyle title.

"I do believe that I'm set up well for worlds," Manuel told SwimSwam after clocking 53.65 in June.

"I've been faster in-season in my events than I was last year and training's been going really well, so I'm pretty confident with how I'll swim at worlds," added the two-time Olympic champion.

"But also just looking forward to swimming well in 2020 at Olympic trials and Tokyo.

"Obviously I want to swim really well at the world championships, but it's kind of also a stepping stone to see where I need to be in 2020."