Poland's top-ranking banker in North America defected to the United States almost three months ago, bringing with him what one source described as "extraordinary" information about East European intelligence.

Sources said that CIA and FBI agents still are interviewing Andrzej Treumann, who came to New York three years ago to open the North American office of Bank Handlowy, the foreign trade bank handling most of Poland's $26 billion debt to western banks and governments. Treumann had been responsible for negotiating the new schedule of Polish payments on the debt.

"While he was a banker and not a spy, Treumann apparently had been used quite often by Polish and East European intelligence agencies," one source said. "We are getting extraordinarily high quality information about East bloc intelligence methods from him."

Said to be in his early 40s, Treumann, his wife and daughter now are in the Washington area where he has been talking freely with CIA and FBI agents about what Polish intelligence sought from him and what he gave them.

Sources said they did not know Treumann's reason for defecting, although one source hinted that he sought political asylum here because of the crackdown on the Polish people by the Polish military government. Several high-ranking Poles have defected to the United States since martial law was declared in Poland, including Ambassador to the United States Romuald Spasowski and Ambassador to Japan Zdzislaw Rurarz.

The New York Times said yesterday that Treumann stopped going to his Park Avenue office late in July and that he and his family vacated their Queens apartment early in August. Reports also began circulating in the New York banking community that he had vanished, although he told friends he was going back to Poland in August.

In late August, the Times reported, Treumann's bank mailed a one-sentence notice to U.S. banks saying Treumann "terminated his activities as our representative in the United States." The telephone in Treumann's apartment was answered by a man with an East European accent who said: "Mr. Treumann doesn't live here any more. Good night."

Before opening Bank Handlowy's New York office, Treumann had been a senior official of the bank in Warsaw where he was the equivalent of a senior vice president of an American bank and where he helped arrange some of the biggest western loans to Poland in the 1970s.

Poland still owes $26 billion to the West, including $1.6 billion to the U.S. government and $1.4 billion to U.S. banks.