Penn was speaking at a news conference after talks with Japanese government officials on issues related to the discovery of banned spinal material in a US beef cargo on Jan. 20, which led Tokyo to stop all imports of the meat from the United States.
'It's a little premature to talk about a timeframe, but we do want to move as expeditiously as possible,' Penn said.
He said the United States needed to continue its investigation into how the incident happened.
'We're still trying to ascertain facts in some cases,' Penn said.
At a separate news conference, a Japanese Foreign Ministry official told reporters that Japan had urged the US side to find the cause of the problem over the imported material and prevent it recurring.
The suspension came just a month after Japan lifted a two-year ban imposed after the discovery of mad cow disease in the United States in December 2003, which had halted annual trade worth about $1.4 billion.
Formerly the top US export market, Japan agreed to resume imports last month but it imposed certain conditions, including the removal of all risk material that could cause the feared brain-wasting disease.
Penn said he believed the shipment with the banned spinal material was an isolated case, as the US plant involved had only had limited previous experience in international trade.
The USDA has barred the Brooklyn firm that exported the shipment from making further exports to Japan.
'(US officials) explained to us that the matter was caused by human error according to the information currently available to them,' the Foreign Ministry official said.
Information wanted
Senior Japanese government officials have said the United States must provide an explanation of how the banned spinal material came to be found in the shipment before it will allow US beef imports to resume.
Meanwhile, President George W. Bush said that the United States will be aggressive in reopening markets to American beef and assuring foreign buyers that there is no danger of mad cow disease.
Bush said he wanted to 'make sure we're treated fairly' in beef exports.
Many countries banned US beef after the discovery of mad cow disease there in 2003, dealing a heavy blow to the US beef industry, whose exports used to account for 10% of output.
Experts say Washington must act quickly to re-establish beef trade with Japan to prevent a possible spillover to neighboring countries.
The US Agriculture Department has lobbied furiously to rebuild billions of dollars in beef trade since 2003 after many countries halted imports due to mad cow disease, formally known as bovine spongiform encephalopathy (BSE).
Countries that followed Japan in easing bans on US beef include South Korea, once the third-largest overseas market, and Hong Kong, previously the fifth-largest.
Both remain open to US beef imports.
As Washington worked to try to contain the issue, Japan said it has confirmed that a 64-month-old cow that died last week in northern Japan had mad cow disease, its 22nd case.
Many Japanese consumers remain concerned about Variant Creutzfeldt-Jakob disease, the human version of BSE, which experts say is caused by eating contaminated beef.
The disease has been blamed for more than 150 deaths worldwide, including one in Japan. (Reuters)
