TOKYO, May 9 (Reuters) - Brain matter carrying mad cow disease from the two youngest cattle confirmed with BSE in Japan has so far failed to infect mice in tests, a Health Ministry official said on Wednesday.

The test results could influence Japan's trade talks with the United States, as Tokyo has restricted American beef imports to cattle aged 20 months or younger on grounds that the youngest case of the disease was found in a 21-month-old animal.

Washington is pressing Tokyo to raise the limit to up to 30 months, arguing this is in line with international standards.

Japanese scientists used brain matter from cattle aged 21 months and 23 months which had been diagnosed with mad cow disease in 2003 and were subsequently slaughtered.

The brain matter was injected into mice, but the animals did not show signs of developing the disease.

"The results so far do not show that (mad cow disease) has spread," the Health Ministry official said.

He did not specify when the tests started.

The official said he expects the test results to be eventually presented to the Food Safety Commission, a panel of experts that assesses risk and makes policy recommendations.

The Japanese animals whose brain tissue was used for the experiment are believed to be some of the youngest cases of bovine spongiform encephalopathy, or BSE, found in cattle in the world.

Experts believe older cattle are more likely to develop the brain-wasting disease.

The United States is still struggling to regain its previous role as one of the top foreign suppliers of beef to Japan, a position it lost after it reported its first case of mad cow disease in December 2003, prompting Japan to ban the meat.

It has since resumed exports, but beef sales have been slow in Japan due to a number of reasons including the restriction on the cattle's age.

The World Animal Health Organization is expected to give the United States a "controlled risk" rating later this month, a ruling Washington is expected to use to pressure Japan to further open its beef market.

Japan was the top foreign beef market for the United States before the ban, buying some $1.4 billion worth of the meat in 2003.

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