TOKYO — In unwelcome news for U.S. farmers, Japan is slapping emergency tariffs of 50 percent on imports of frozen beef, mainly from the U.S.

Finance Minister Taro Aso announced the move Friday, saying he was prepared to explain the decision to the U.S. side.

“The tariff will take effect automatically as the volume of the imported U.S. frozen beef exceeded the quota set by law,” Aso said, “So this is what has to be done.”

The usual tariff rate for frozen beef imports is 38.5 percent. Under World Trade Organization rules, Japan can introduce safeguard tariffs when imports rise more than 17 percent year-on-year in any given quarter.

Beef imports have risen quickly while prices fell as the U.S. livestock sector recovered from drought.

But the U.S. livestock sector faces stiff competition from Australia, whose free trade agreement with Japan means it does not face the same emergency tariffs.

Relatively affordable “Aussie beef” is an increasingly popular feature of most supermarket meat sections.

The U.S. and Australia account for 90 percent of all imports of frozen beef, which is mostly used by beef bowl, hamburger and other fast food outlets.

The Finance Ministry reported 89,253 metric tons of frozen beef were imported so far this year.

But frozen beef accounts for only about of fifth of Japan’s total beef imports.