The news has prompted shops and restaurants to ditch American beef

Robert Zoellick made the comment during a meeting with Japan's Agriculture minister in Tokyo.

On Friday, outlawed spinal material was found in a 390kg batch of veal from New York triggering the new ban.

The move came months after a two-year embargo on US beef - imposed over fears of BSE (mad cow disease) - was lifted.

Tokyo had lifted a ban on imports of cows aged 20 months or younger as long as parts thought to have a higher risk of carrying BSE - such as brains, spines and bone marrow - were removed.

'Sincere regret'

Following the hour-long meeting with Mr Zoellick, Japan's agriculture minister Shoichi Nakagawa told reporters he had called on the US to make stronger efforts to ensure conditions on imports were not violated again.

"The US has a commitment to Japan regarding beef exports and that is a commitment that we take seriously," a US Embassy official quoted Mr Zoellick as saying.

The official also added Mr Zoellick had expressed "sincere regret" about the situation.

He also revealed that the US Department of Agriculture has launched an investigation and removed the company involved from its export programme.

Meanwhile a team of experts will arrive in Japan on Monday to meet Japanese officials in an effort to persuade Tokyo not to reimpose a total ban on imports.

The previous ban halted trade worth some $1.4bn a year - as the Japanese market is one of the biggest for American beef.

BSE was found in US cattle at the end of 2003, prompting Japan to impose the import ban.

Eating beef from infected cattle can cause a fatal human brain disorder, variant Creutzfeldt-Jakob disease.