Washington Bancorp, parent of the National Bank of Washington, has become the latest in a series of regional banks to boost their loan-loss reserves to cover troubled loans to less-developed countries.

The company said it added $30.6 million to loan-loss reserves in the fourth quarter to go along with a previously announced $12 million provision in the second quarter for potential losses from Latin American loans.

The action means that the Washington bank's total loan-loss provision for 1987 was $46 million and that it has now set aside 50 percent of its Third World exposure for losses.

The immediate result of increasing the reserves was to force the company to report a loss of $25.6 million for 1987 after a profit of $8.2 million ($1.64 per share) in 1986.

In the fourth quarter, the company lost $20.1 million, compared with a profit of $2.6 million (52 cents) in the same period of 1986.

Numerous other regional bank holding companies, including Signet Banking Corp. and Crestar Financial Corp. in the Washington area, recently have boosted reserves for Third World debt in an effort to put the problem behind them.

In announcing Washington Bancorp's decision, Chairman Luther H. Hodges Jr. said the decision "was made to follow the practice by many leading banking institutions of establishing a loss reserve of 50 percent for all nontrade-related debt in Latin America.

"These actions reflect the realities of the marketplace, and strengthen our earnings for the future," Hodges said.

A spokeswoman added that the bank believes that it now has adequate reserves to cover possible losses from loans to less developed countries.

Bell Atlantic Corp. said 1987 profit rose 6 percent, to $1.24 billion ($6.24), from $1.17 billion ($5.85) in 1986. The company reported operating revenue of $10.3 million for the year, compared with $9.9 billion in the year before.

The Philadelphia-based regional telephone company, whose subsidiaries include Chesapeake & Potomac Telephone Cos., said it earned $285 million ($1.44) in the fourth quarter, up 5 percent from $271.9 million ($1.37) a year ago.

Operating revenue in the quarter was flat at $2.6 billion.

"With our 1987 results, we have achieved our objective of improving year-over-year earnings in the 6 to 8 percent range," said Bell Atlantic Chairman Thomas E. Bolger.

"These earnings reflect the strength and vibrancy of the economy in the middle-Atlantic states, as well as the increasingly important contributions of our enterprise companies" -- divisions that are working on such new, growing ventures as cellular telephone communications, computer maintenance and financial services, Bolger said.