The Reagan administration for the first time is willing to adapt its strategy calling for the Third World debt to be handled on a "case by case" basis, a senior Treasury official said yesterday.

In briefing reporters on U.S. positions on issues to come up next week at the World Bank and International Monetary Fund joint annual meeting in Seoul, the official said that the administration "is reviewing debt strategy."

Meanwhile, a House subcommittee dealing with the international debt crisis urged the administration to take a bolder approach by strengthening the World Bank and the other multilateral development banks.

The debt crisis is sending dangerous ripples "onto our own domestic banking and commercial shores," said a report by a House Banking subcommittee headed by Rep. Stan Lundine (D. N.Y.).

Heretofore, the United States, along with the World Bank and IMF, have insisted that the problems of each debtor country be considered separately, on a "case-by-case basis." Others have suggested that the common underlying problems in the Third World countries deserve a unified, global approach.

The Treasury official said that the U.S. strategy "is still case by case," but that it is essential that the less developed countries (LDCs) be placed on a path leading to economic growth, "and not just austerity."

"Considerable progress has been made in dealing with this problem," he said. "But there are certain problem areas within the debt strategy, one of which is that commercial banks are not providing adequate net new financing."

He added that, in large part, the LDCs are responsible for achieving growth themselves, especially by encouraging private investment. But he also made clear that the administration is preparing new initiatives to bring the World Bank more fully into the picture.

"The bank should be more involved in the debt-strategy exercise," he said.

But he cautioned, "There is no notion of the bank taking over, or of downgrading the IMF. The fund remains a cornerstone of that debt strategy, and will continue to occupy a position of very, very great importance."

The African countries, he said, "need sort of a more broad based, long-term structural adjustment approach." The fund "is not a development institution, and it seems appropriate to try to find some way to take account of the fact that these countries face longer-term adjustment problems, and to try to work out some cooperative basis between the bank and the IMF."

The Treasury also is known to be concerned that unless the World Bank role is beefed up, the growth that is supposed to follow the IMF's austerity programs in Latin America won't show up quickly enough to encourage political stability.

Nonetheless, the official said that, at Seoul, the United States will not be prepared to approve a general capital increase (GCI) for the bank, as has been suggested by World Bank President A. W. Clausen. "Decisions on the size and timing of a possible GCI commitment would depend upon the quality and the quantity of lending that emerges in the near term," he said.

He added that the bank could lend more money on its existing capital base if it operated on a more flexible basis.

"It may be that there has to be internal review of policies in the World Bank, changes made in the efficiency of its resource use, and so on," the official said. "There's much to be done before a funding requirement has to be faced. . . . "

Other policy positions announced by the official:

*The United States has not yet taken a position on a new grant program for the International Development Association, the bank's concessional aid arm. He would not rule out the possibility that there would be no IDA-8 program, but said that this extreme position was "not a high probability."

*In the possibility of a new issue of Special Drawing Rights (SDRs), the IMF's international currency, the United States believes the proper "criteria" to authorize such an issue have not been established.

*The United States favors a modest reduction in the "access limits" that govern how much money individual countries may borrow. Ultimately, the United States wants to phase out these extra borrowing rights established in 1981.