The United Nations secretary general appears to have softened his predecessor’s criticism of Iran last year over its missile tests, a volatile issue in Iran’s relationships with other powers, including Israel and the United States.

The milder language is contained in a report by the secretary general, António Guterres, to the United Nations Security Council that has not yet been released. A softening of the criticism would be significant partly because the United States has called Iran’s missile tests unacceptable.

The Trump administration imposed sanctions on Iran in February and May in response to what it described as “bad behavior” with respect to the tests.

Mr. Guterres’s relatively mild language in a passage of the report concerning those tests could complicate any American-led effort to further penalize Iran for them at the United Nations. A copy of the report, dated June 14, was seen by The New York Times on Wednesday.