Riggs National Corp. announced yesterday that it expects to report a loss of about $12 million for the second quarter because of its decision to increase its loan-loss reserve to cover potential writeoffs and delays in collecting Third World debts.

Riggs said it will add $32 million to its reserves to cover potential loan losses from loans to 16 lesser-developed countries, bringing its loan-loss reserve to $71 million, or 2.4 percent of total loans. About $52 million of the loan-loss reserve, or 38 percent of the reserve, relates to problem country loans.

Riggs also put its $37 million worth of loans to Brazil on a nonaccrual status because that country has declared a moratorium on paying interest on its debts. That will mean $1.6 million less income during the second quarter than was originally anticipated.

The two actions will result in losses of $12 million for the quarter and $1.6 million during the first half of 1987 for Riggs, which had a profit of $10.4 million during the first quarter of this year.

Riggs officials said they expect to report a profit for the full year.

Chairman Joe L. Allbritton said: "Since 1981, Riggs has reduced its problem country loans and increased its reserves to guard against problem country exposures. The additional provision announced today will allow us to maintain our position of leadership by allocating reserves to our problem country exposures in amounts which approximate the recent market discount on those loans."