Jessicah Schipper has claimed the butterfly double at the Pan Pacific Swimming Championships after taking out the 100 metres.

Schipper, 19, set a new Pan Pacific Championship record of 57.30 seconds, to take over half-a-second off the previous record of 57.88, held jointly by Americans Jenny Thompson and Natalie Coughlin.

It was the third fastest time of Schipper's career and the eighth fastest time in history and came just two days after she broke the world record in claiming the 200m butterfly on the opening night.

Schipper said Inge de Bruijn's world record of 56.61 was now on her radar.

"It's definitely one of my goals that's for sure but to swim that fast everything has to be perfect and there are always things you can improve on," she said.

The Australian's win came on a day when two world records were broken by Americans.

Aaron Peirsol reaffirmed his place as an unbeatable force in the men's 200m backstroke with a time of 1min 54.44sec, breaking his own world record by more than two-tenths of a second.

Then, led by multiple Olympic gold medallist Michael Phelps, the US men's 4x100m freestyle relay team broke the world record with a time of 3:12.46.

Phelps, Neil Walker, rising sprint star Cullen Jones and veteran Jason Lezak broke the previous record of 3:13.17 set by South Africa in winning gold at the Athens Olympics.

Canada took the silver in 3:16.12, ahead of the Australian combination of Eamon Sullivan, Andrew Mewing, Leith Brodie and Kenrick Monk) who clocked 3:16.42 for third.

Walker said the Americans were determined to bounce back after a disappointing result at the 2004 Games.

"We said 'Hey, let's bring this back home, let's bring the record back home to the US where it belongs," he said.

The relay victory capped a night that had started out disappointingly for the United States, when South Korean upstart Park Tae Hwan posted a come-from-behind upset over American favourite Klete Keller in the men's 400m freestyle.

Park, who turns 17 in September, clocked 3:45.72 to claim the first gold medal for a South Korean swimmer at a major international meet.

The Australian women's team of Shayne Reese, Kelly Stubbins, Linda MacKenzie and Melanie Schlanger were third behind the US and Canada in the 4x100m freestyle relay.

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