UNESCO has failed to make the changes that would have been required for the United States to remain in that beleaguered U.N. organization, according to the senior State Department official responsible for monitoring international organizations.

"Those changes have not occurred," Gregory J. Newell, assistant secretary of state for international organization affairs, said. "We've been working with UNESCO unsuccessfully for three years."

Newell said in an interview Monday that he expects the United States will now go ahead with its decision, announced at the end of 1983, to withdraw from the U.N. Educational, Scientific and Cultural Organization on Dec. 31.

"Any chance of remaining in the organization would appear to be slim," he said, although a formal decision was still awaited from the White House.

Some congressmen are still hoping that the administration will delay the withdrawal. Rep. Jim Leach (R-Iowa), ranking minority member of the House subcommittee on human rights and international organizations, said yesterday at a press conference with representatives of nongovernmental organizations fighting withdrawal: "To refuse to stay and fight corruption from within UNESCO is a denial of international responsibility. It may be a form of corruption itself."

Newell said discussions continued among administration officials as to what steps should be taken beyond the withdrawal from UNESCO. He said no recommendation had yet been sent to the White House for President Reagan's approval. But other officials said the administration was on the verge of making an announcement and that much of the discussion now concerned how to "package" it.

Newell said support for withdrawal from UNESCO from a wide range of top U.S. government officials is now stronger than when the initial decision was made last year.

"They've been out and around and they've heard the complaints about UNESCO," he said.

On Nov. 28 an interagency group of senior officials headed by Undersecretary of State Michael Armacost held what may turn out to have been its last meeting on the subject of UNESCO. More than half a dozen government agencies have been participating in the discussions of what to do about UNESCO.

Newell did not shut the door completely to an eventual U.S. reentry into the organization. He said it could be argued that a U.S. withdrawal, combined with Britain's recently announced decision to withdraw a year from now, would add to pressures on the organization to reform.

The Reagan administration plans to establish a mechanism such as an observer mission that will continue to monitor UNESCO activities once the United States leaves.

Britain's decision to withdraw caused speculation in Paris, where UNESCO has its headquarters, that its director general, Amadou Mahtar Mbow, might be forced to resign to prevent the United States and other western nations from leaving.

But Mbow has rejected resigning and Newell denied rumors that the United States had a "hidden agenda" aimed at forcing Mbow out.

"Our criticisms of UNESCO deal with substance and not with personalities," Newell said. "We've been asked by other governments, 'If Mbow were to go, would we reconsider our decision?' and the answer is emphatically no."

Newell said that the United States, which finances 25 percent of the UNESCO budget, had substantive complaints about the organization in four major areas.

He charged that UNESCO had "politicized" the subjects it dealt with. UNESCO, he said, continued to show an affinity for "statist solutions" to problems. It had failed to protect the views of major budget contributors that were in a minority in the 162-member organization, he said. And finally, he added, while its executive board had passed a resolution calling for a zero-growth budget for two years, it had added a "troubling footnote" that would allow the director general to detail specific programs for the poorest countries, amounting to 2 percent of the budget.

Newell acknowledged that UNESCO had made "behaviorial changes" that were encouraging to the Reagan administration. He said these included "less offensive language" about western nations.

He argued that the move to withdraw from UNESCO had had a "salutary" impact on other international organizations, causing them to tighten their budgets and to engage in less politicized activities.

But supporters of UNESCO in this country argue that U.S. withdrawal would damage important international programs, such as UNESCO's efforts to bring literacy to hundreds of thousands of children in poor nations who have no access to a formal education.

State Department officials say that the United States intends to continue such programs through its own means, but congressional critics question whether the State Department has the capability to do this.