A number of the nation's major regional banks this week boosted their reserves for losses on loans to Third World countries, in effect acknowledging that they may never collect on as much as 60 percent of their loans to them.

Industry officials said the latest round of additions to reserves, the second of 1987, was prompted by the regional banks' desire to put the problem behind them and the feeling that they could do so without unduly eating into their capital bases.

It was unclear whether major money-center institutions, which would find it more difficult to further boost their reserves for troubled Third World loans, would follow suit, analysts said. Yesterday, two of these banks, Chemical New York Corp. and the Bank of New York Co., released year-end earnings that revealed no major additional reserves.

One of the regional banks that increased its reserves was Signet Bank, a large Richmond-based institution. "We had seen a continuation in the deterioration of the market value of the LDC {less developed country} credits since the action we took last June," said W.B. Millner, Signet's executive vice president.

This week, Signet announced it was raising its loan-loss reserves to 59 percent of its Third World loans, its second major increase in reserves this year.

"No one was holding a gun to our head, but we felt that realistically we would have to downgrade the value {of the loans} at some point," Millner said. "We felt that it would be better to do it in 1987 and get it behind us, than run the risk of doing it in 1988 and ruining two years."

This week, several major banks took similar steps to cover Third World loan losses in the fourth quarter, including Continental Illinois Corp., First Interstate Bancorp, National Westminster Bank USA, as well as a variety of other regional banks ranging from Indiana National Corp. to Southeast Banking Corp. of Miami. The largest bank to boost its reserves significantly in recent weeks was Security Pacific Corp., the country's seventh-largest bank holding company, which added $350 million to cover losses on troubled loans to Latin America and the Philippines.

Bank analysts are anxiously awaiting the year-end results of the major New York money-center banks, most of which are scheduled to be released next week. These banks face steep cuts in earnings and capital if they boost reserves, and they will be much more reluctant to concede further losses on troubled LDC loans, according to analysts.

"Some will. Some won't. Some of the decisions are still being made," said James J. McDermott Jr., director of research for Keefe, Bruyette & Woods Inc. in New York.

James H. Wooden, a bank analyst for Shearson Lehman Bros., termed the wave of regional bank action this week on LDC loans a "gigantic case of sticking it to the money-center banks. ... What is really behind the regional bank thing is {an effort} to distinguish themselves in the minds of investors."

Yesterday, Chemical announced a year-end loss of $853.7 million, compared to profits a year ago of $402.4 million ($7.57 per share) -- largely reflecting the previously announced $1.1 billion added to reserves for potential LDC loan losses in its second quarter.

Bank of New York reported a 33.4 percent drop in its profits to $103.4 million ($2.81 per share) from $155.2 million ($4.54). The bank added $135 million to its reserves in the second quarter, but only added $7.2 million in the fourth quarter. The bank has now set aside for losses about 33 percent of its $447 million exposure in the Third World, according to Owen A. Brady, a Bank of New York vice president.