NEW YORK, JULY 11 -- Federal regulators have ordered the nation's largest banks to write off sizable portions of loans to economically troubled Brazil and Argentina, bankers said today.

The order, made in letters from the Interagency Country Exposure Review Committee, will require banks to write off an additional 20 percent of $2.9 billion in outstanding loans to Argentina and 20 percent of $11 billion in loans to Brazil.

The write-down of Brazilian loans is the first required by federal authorities. The move on Argentina boosts to 60 percent the amount of loans that must be written off by the 36 affected U.S. banks.

Bank officials, speaking on condition of anonymity, confirmed the receipt of letters in recent days from the committee, which consists of three federal bank regulators. The committee normally does not announce its decisions.

The action could force some of the banks to further boost reserves to protect against losses from nonperforming loans to developing nations. If a bank does not have enough set aside in reserves to cover the write-downs, the new charges could depress earnings.

The 20 percent write-off of Argentine debt was the third ordered by federal authorities.

The write-off of Brazil's debt was considered surprising because the country has undertaken economic reform under new president Fernando Collor de Mello and is expected to resume negotiations this year on meeting some bank debt obligations.