Both the International Monetary Fund and the World Bank are moving to provide more help for highly indebted developing nations.

How much impact the coming IMF-World Bank initiatives will have on the persisting Third World debt problem is unclear, however.The two international agencies will revise their lending rules somewhat.

The World Bank, for example, may extend grace periods and maturities on its loans and eliminate a 0.5 percent commitment fee on International Development Association credits.

World Bank President Barber Conable is expected to make such proposals next Thursday to the bank's executive board.

The board is considered likely to consider in coming months other initiatives, particularly how the bank might help foster debt swaps between developing nations and creditor commercial banks, to help reduce debt burdens.

The IMF's interim committee, a 22-member ministerial panel, Friday appeared to endorse suggestions to let the IMF extend both longer and bigger loans for developing nations undertaking economic reforms.

It approved proposals for the IMF to help cushion these countries from the effects of both higher international interest rates and import prices and other adverse economic forces beyond their control.

Michel Camdessus, the IMF's managing director, said that this new contingency fund might be created within a matter of weeks.

In a spirited defense of his agency, Mr. Camdessus said that charges that the IMF is taking more money from the developing nations than it is providing are misleading.

The bulk of the IMF's incoming money represents loan repayments from countries having greatly improved payments conditions - India, Korea, South Africa, Thailand and Turkey, he said.

Among the IMF's lending policy changes, signaled by the interim committee as likely, are lower interest rates and longer maturities for the agency's medium-term Extended Fund loans.

These loans, it said, might be lengthened from a maximum three to four years.

The IMF, it suggested, might also, in certain cases, agree to increase the amounts that countries borrow from this fund.

The interim committee directed the IMF's executive board to report by April 30 next year on a possible boost in the IMF's lending resources.

It confirmed, however, that a new issue of IMF special drawing rights, an international reserve asset, is not in prospect, even though most nations favor it.

The committee, for the first time, formally noted the problem of IMF arrearages.

It asked the IMF executive board to report on measures to reduce and eventually eliminate borrower nations' arrears to the IMF, which a year ago had already reached $1.5 billion.