Currency traders sent the dollar sharply higher again yesterday on the strength of an unconfirmed report of a secret agreement by the United States, Japan and West Germany to pump up to $15 billion into the markets to stabilize the U.S. currency.

According to the report, which officials of the three nations would neither confirm nor deny, the agreement called for each nation to contribute to a $15 billion pool that would be used to buy the dollar to keep its value between 130 and 140 Japanese yen.

The report, carried on the Japanese state-run television network, touched off a hectic buying wave. The dollar soared to 131.20 yen from 126.88 earlier in the day, and to 1.6615 German marks from 1.6290.

The sudden dollar surge sent speculators scurrying to cover short positions -- buying dollars to make good on prior commitments. Meanwhile, central banks further increased demand for the dollar through their purchases, escalating prices. Despite profit-taking late in the day, the dollar held on to most of its gain, closing at 129.00 yen and at 1.6440 German marks in New York.

For the third day in a row, the strong dollar helped buoy stock prices. On the New York Stock Exchange, the Dow Jones average of 30 industrials closed 6.30 points higher at 2037.80. {Details, Page F1.}

A senior Reagan administration official briskly responded "No comment!" to the report of a secret agreement. But he suggested that the strength of the dollar in the past three days represented a belated recognition that the Group of Seven western industrial nations were serious in their Dec. 22 commitment to halt the dollar slide.

"It's my honest belief that the markets were too quick to rush to a judgment that the underlying policy actions {of the G-7} were too little, too late," the official said. The G-7 consists of the United States, West Germany, Japan, Britain, France, Italy and Canada.

Seemingly content to keep currency exchange market speculators guessing, neither he nor any other top official would deny the report of a secret pact. A West German diplomat in Washington said, "I don't know and don't want to know about such things. But even if such an agreement should exist, one shouldn't talk about these things."

The combination of continued dollar purchases by central banks and the Japanese news report seemed to alter basic attitudes in the exchange markets.

Market watchers, convinced for the moment that the dollar hit bottom at around 120 yen and 1.57 German marks on Monday, began to wonder just how high the G-7 ministers had agreed the dollar might safely go. The reported "Big Three" accord suggested an upper limit of 140 yen, but did not mention the German mark.

"I think they {the G-7 ministers} are trying to convince the exchange market that there is a lot of room for the dollar to move on the upside, so that the market won't have a one-way bet selling dollars," said C. Fred Bergsten, director of the Institute for International Economics.

The Japanese broadcast said the three nations acted to prop up the dollar after the G-7 statement on Dec. 22, endorsing stability, had been ignored by the exchange markets. A declining dollar could lead to greater inflation and higher interest rates, dampening economic growth.

Financial markets also took note of other bullish statements from Tokyo. Bank of Japan governor Satoshi Sumita suggested that the dollar still had room to go up, saying: "I don't think the dollar's strong rebound {through Tuesday} is excessive."

And Toyoo Gyohten, Japanese vice minister for finance, added that the G-7 had pledged not merely to stop a further decline of the dollar, but to push it higher.

The market reading of the Gyohten and Sumita statements and the broadcast report was that the G-7 intended to drive the dollar up toward 140 yen with sustained intervention -- entering the market to buy dollars.

West German Finance Minister Gerhard Stoltenberg, in an interview with the newspaper Die Welt, echoed the American senior official's comment that the markets had underrated the determination of the seven industrial nations.

Only lately, he said, had markets begun to appreciate "the full impact" of the G-7 agreement. He added that the United States is "really serious" in its view that dollar stability is in the common interest of all major powers.

"Within the American government, a conviction has grown that a further dollar decline also hurts the interests of the United States, can bring about inflation dangers, and can create problems for the world economy," Stoltenberg said.

Bergsten said that the United States, interested in improving the competitive position of American exports, will be careful about letting the dollar stay above 130 yen for the long run.

A higher dollar tends to make American exports less competitive, and gives an edge to imported products.

"As a tactical move, I don't think anyone would be leery of {letting the dollar} run up to 140 yen or beyond, but they wouldn't let it stay there," Bergsten said.

Exchange market participants seemed dubious about the impact of a $15 billion intervention package, however.

They pointed out that at the level of recent curency market interventions, reportedly amounting to $2 billion or more a day, $15 billion did not appear overwhelming.

But Bergsten noted that there could be some psychological benefit in keeping currency speculators guessing.

Trading yesterday underscored that point.

"The Treasury often keeps the Fed on a short leash when it comes to intervention," according to Bergsten.

"If there's agreement in advance that there's $5 billion in U.S. intervention money available, it might provide more flexibility for the intervening authorities than they would have on standard operating procedures."