The bitter dispute over the Reagan administration's proposal to gain near-veto power over loans made by the Inter-American Development Bank is beginning to reduce the amount the bank can lend to poor nations, officials said yesterday.

Meetings between Treasury officials and bank governors on Tuesday and Wednesday made no progress toward resolving the disagreement, which also was the subject of an unusual meeting between seven Latin American ambassadors and three members of Congress this week and has generated several letters from Latin American officials protesting the U.S. stance.

Mexican Finance Minister Gustave Petricioli said in a letter to Treasury Secretary James A. Baker III earlier this month that major debtor nations will soon be paying more money back to the IDB than they receive in new loans if higher funding for the bank continues to be blocked by the dispute.

"It is my impression that delaying the IDB negotiations is in no one's interest and defeats the program for sustained growth you announced in October 1985 in Seoul," Petricioli said. "Experience has shown us that regional developing countries, currently engaged in structural adjustment, depend to a great extent on the financial resources of the multilateral banks, such as the IDB."

Since last year, Baker has been at odds with the bank and the Latin American governments that make up most of its membership over Baker's proposal to change the way the bank approves loans. Loans for development now can be approved by a simple majority of the bank's members.

Baker first proposed allowing new loans -- which the administration contends often are made for dubious projects without sufficient credit examination -- to be vetoed by a 35 percent vote. The United States holds 34.5 percent of the voting shares; with Canada, which usually votes the same way, it thus could effectively have vetoed loans.

There have been various proposals for compromise on both sides. The administration's most recent plan, suggested last March, would allow loans to be vetoed by a 40 percent vote and delayed for up to 18 months by a 35 percent vote.

Latin American nations contend that they cannot agree to change the voting rules without approval by their parliaments, a politically difficult task. This week, bank governors from Latin America, meeting in Washington, floated a plan to approve loans in two phases, one of which would permit a delay with 40 percent of the votes. This would allow the United States and Canada to delay a loan if they could persuade one European nation to go along.

Deputy Assistant Treasury Secretary James W. Conrow said the United States rejected the proposal, which he said "fell far short of the secretary's compromise." The issue will be discussed again when the bank's governors meet in Guatemala in mid-October.

Conrow denied there was any contradiction between the Baker Plan for dealing with developing-nation debt, which calls for $29 billion in new public and private lending to those nations, and the IDB dispute, which has hampered its ability to lend.

"We are committed to the principle that the IDB could play a bigger role in the Baker Plan if we get the reforms, but we are finding Latin America is less interested than we thought," Conrow said. He said the bank has been slow to get new projects ready for loans, and will have $10 billion in lending resources over the next four years even if no new funding is approved.

That translates into about $2.5 billion in money that can be lent in 1987, slightly less than last year and considerably less than the $5.5 billion or so in annual lending possible if the dispute could be solved. Baker has said he is willing to increase the U.S. contribution to the IDB if his proposals are accepted.

Members of Congress, meanwhile, are becoming interested in the dispute. Early this week, the ambassadors of seven Latin American nations, including Mexico, Brazil, Argentina, Venezuela and El Salvador, met with three members of the international development subcommittee of the House Banking Committee at the legislators' request.

"The ambassadors made clear that they had already accepted a number of policy and structural reforms having to do with sectoral lending. But they said the notion of a U.S. veto was unacceptable," said Rep. Bruce A. Morrison (D-Conn.), one of the legislators at the meeting. The others were Del. Walter Fauntroy (D-D.C.) and Rep. Joseph Kennedy II (D-Mass.).

Although members of Congress are unlikely to do more than meet with administration officials and perhaps hold hearings, Morrison indicated that there was unhappiness with the administration position.

"I think they {the Latin Americans} are offering compromises, and within their offers additional adjustments can be made," he said. "The U.S. can get more stringent review of the terms of lending without destroying the multilateral character of the institution, which is what would happen if the U.S. approach were taken."