Treasury Secretary Donald T. Regan said today that the world debt crisis is being aggravated by the withdrawal of regional and smaller banks from lending to underdeveloped nations.

Speaking to reporters traveling with him to Manila, where he is to attend the annual meeting of the Asian Development Bank, Regan also said he expects to hold more conversations in the International Monetary Fund and elsewhere about a possible overhaul of the world money system, but cautioned, "I suspect it's going to take us a couple more years before we've thought this thing through."

Some economists have been calling for a new Bretton Woods to replace that system of fixed exchange rates that broke down in 1971 when former president Nixon severed the link between the dollar and gold.

Regan said it is "understandable" that banks--especially small ones--are wary of lending overseas. But he said that their withdrawal worsened the outlook for world trade and recovery. "We have this problem of banks pulling back and not wanting to finance as much trade . Now if there's not going to be as much financing, how do you keep world trade going?" he asked.

Trade and debt will be up for discussion at the economic summit in Williamsburg, Va., at the end of this month.

Regan, who has rejected the idea of stepping up intervention in the currency markets to lower the value of the dollar, said that he expects the dollar to decline in coming months as U. S. interest rates fall further, reducing the dollar's attractiveness.

He stressed the connections between recovery in the world economy and world trade and the debt problems that are preventing many developing nations from buying so much from the United States and others. "I think there are ways that the trading system could be improved . . . in the ways you get credit and the like," the Treasury secretary said. However, he cautioned that the United States isn't planning new measures on trade and finance.

Referring to congressional resistance to voting money for foreign nations, he said "it would be difficult indeed to expect the United States to be more forthcoming." Moreover, a bill now going through Congress that will stiffen supervision of bank lending overseas means "we can't lean too much" on banks to lend more, he said. "We have to allow them to run their own bank in their own fashion."

But Regan added that, because the commercial banks themselves have caused a "shortfall in some of the negotiated settlements that the United States, the IMF and the others--including the commercial banks--have worked out" with debtor nations, it is up to the bankers to "solve this problem among themselves."

Brazil, with more than $80 billion of foreign debts, is most affected by the failure of banks to put up all the money agreed to in negotiations. But Regan said there had been some pulling back by small and medium-sized banks in almost all of the debt renegotiations that have taken place since the crisis arose last summer.

The basis for these renegotiations is typically that all the commercial banks involved put up a proportional share of the new money that is needed. But while the big banks are virtually forced to keep lending to protect their huge outstanding loans, many smaller banks have wanted to cut their losses and get out.

Finance ministers of the five largest industrial nations agreed last weekend to discuss more frequently in the coming year how to bring their economies more into line, officials traveling with Regan said.

After last year's economic summit in Versailles, the nations agreed to monitor disparities in their economic performance with the help of the International Monetary Fund to bring policies more into line and thus provide conditions for more stable currencies.

At a meeting of the five--the United States, France, Germany, Italy, and Japan--in Washington last weekend, the IMF gave a "forthright and candid" critique of how the nations have performed so far, Regan told reporters en route to Manila.

He said that that the IMF criticized the United States for overly large budget deficits, but was complimentary about U.S. success in reducing inflation. Regan added that "France is the one that differs here" from the generally improved inflation rates.

The United States has been blamed by its allies for pushing up interest rates worldwide and refusing to take steps to lower the dollar. However, officials said Regan now is hopeful that there will be more convergence between economies in the future, with the United States and others now recovering, officials said.