HONG KONG (AP) _ Four Asian countries already reeling with financial difficulties received another blow Monday when a leading credit-rating agency downgraded their debt.

Moody's Investors Service put three of the countries _ Indonesia, South Korea and Thailand _ in the junk-bond category, a step that could make it even more difficult for those countries to borrow money.

Some banks and investors refuse to lend money to or buy bonds from borrowers listed in the ``junk'' or speculative-grade rankings.

Of the four nations downgraded Monday, only Malaysia remained in the upper-tier investment grade. The other three have accepted IMF bailouts totaling nearly $100 billion, but Malaysia says it can take care of its financial problems by itself.

Flemming Larsen, the deputy chief economist for the International Monetary Fund, told CNBC television Monday that the downgrade for Thailand, Indonesia and South Korea complicates the financial situation and means they will have trouble borrowing money. But Larsen said the IMF is optimistic the countries' finances will turn around in 1998.

Malaysia's bond rating was cut to A2 from A1, still well within the investment-grade ranking. Indonesia, South Korea and Thailand were cut to Ba1, the top level of the junk, or speculative-grade categories.

The outflow of funds from the countries and currency depreciations have added to problems in the banking sector, Moody's said.

In addition, slow economic growth in Japan weakens the prospects of other nations being able to sell more to Japan, and Japanese banks' problems with bad loans limit their ability to keep lending in the region, it added.

The ratings for 11 Thai financial institutions, including Bangkok Bank, the country's biggest bank, were lowered to junk status. The Thai government has permanently shut down 56 finance companies _ half the country's financial institutions _ to cope with a deluge of bad debt that analysts attribute to eccessive lending policies.

Banks were badly hurt when the government abandoned its fixed-exchange rate system in July, causing the baht to drop 45 percent in value against the dollar. Thailand's private sector now owes $67.1 billion in foreign-currency debt _ and $29.1 billion must be repaid in less than a year.

Moody's said that in Indonesia, the currency's fall by more than 50 percent has placed considerable stress on businesses' ability to meet external debt obligations. Business difficulties also are likely to create substantial strains in the banking system, it said.

Malaysia's currency and stock markets also have been among those hardest hit by the region's economic turmoil. Since July, the Malaysian ringgit has depreciated nearly 35 percent.

In South Korea, Moody's said, the downgrading reflects worries that the country's near-term foreign currency financing needs may be greater than previously expected.