Japan is standing by its refusal to import U.S. beef from cows older than 20 months unless they have been tested for mad cow disease, a top government official said Monday. 

Japan and the United States are negotiating a compromise on mad cow testing that is expected to lead to Tokyo lifting a ban on U.S. beef imports imposed last December. 

Japan's Food Safety Commission announced last week that the country could import meat from untested cows 20 months old or younger without endangering public health. Japan had previously urged U.S. producers to test all cows. 

Washington is pushing Japan to raise that bar to 24 months, Japanese news reports say, but Chief Cabinet Secretary Hiroyuki Hosoda said Tokyo would resist that. 

"We've been saying that such methods may not be necessary for cows 20 months old or younger, but ... we have to remember that 21-month-old and 23-month-old cows could be infected," Hosoda told reporters. 

Japan has tested all its domestic cows going to slaughter following several cases of mad cow disease, and has urged the United States to impose a similar system after that country suffered its first case of the disease last year. 

Both sides are eager to strike a compromise to allow the import of U.S. beef to Japan, which used to be the most lucrative overseas market for American beef. Japanese consumers, meanwhile, have grown fond in recent years of U.S. beef, which is much cheaper than the domestic product.