All urine samples from the Paris World Athletics Championships last August will be re-tested after the discovery in the United States of a previously undetectable steroid.

In a phone call from Paris, International Association of Athletics Federations (IAAF) spokesman Nick Davies said the world governing body planned to examine all the samples again.

"The IAAF are going to re-test all samples," he said.

The decision follows an announcement by the U.S. Anti-Doping Agency (USADA) last Thursday that several American athletes had tested positive for a new steroid tetrahydrogestrinone (THG).

The positive samples were found in the re-testing of 350 doping samples from the U.S. athletics championships in June.

"I know of no other drug bust that is larger than this involving the number of athletes we have involved, certainly in the anabolic steroid area," said USADA chief executive officer Terry Madden.

"This is a conspiracy involving chemists, coaches and certain athletes using what they developed to be 'undetectable' designer steroids to defraud their fellow competitors and the American and world public who pay to attend sports events."

A test for the drug was discovered by the IOC-accredited laboratory in Los Angeles after an unidentified man sent them a syringe containing THG.

Madden said the man, who said he was a high-profile coach, called the USADA in June alleging that leading track and field athletes were using the steroid because no test existed.

Davies said the world championships samples could be tested at the Los Angeles laboratory or in Paris if the new testing technology was made available. Any positive tests would lead to disqualifications and two-year bans.