WASHINGTON — A panel of former senior American officials and outside experts, including several who recently left the Obama administration, issued a surprisingly critical assessment of American diplomacy toward Iran on Wednesday, urging President Obama to become far more engaged and to reconsider the likelihood that harsh sanctions will drive Tehran to concessions.

In a report issued by the Iran Project, the former diplomats and experts suggested that the sanctions policy, rather than bolstering diplomacy, may be backfiring. As the pressure has increased, the group concluded, sanctions have “contributed to an increase in repression and corruption within Iran” and “may be sowing the seeds of long-term alienation between the Iranian people and the United States.”

The critique comes as both Israel and Congress are urging the administration to go in the opposite direction, to put a sharp time limit on negotiations and, if necessary, to go beyond the financial and oil sanctions that have caused a tremendous drop in the value of the Iranian currency and sent inflation soaring.

“I fundamentally believe that the balance between sanctions and diplomacy has been misaligned,” said Thomas R. Pickering, who was one of the State Department’s highest-ranking career diplomats and whom the department has called on to head up important investigations, including one into the death last fall of the American ambassador to Libya.

In an interview, Mr. Pickering also contended that Mr. Obama should review the covert program against Iran — which has included computer sabotage of its nuclear facilities — to “stop anything that is peripheral, that is not buying us much time” in slowing Iran’s progress. The report itself, however, says nothing about the sabotage effort, which has been a major element of the American strategy.

Mr. Pickering, who is such a towering figure in the State Department that a major program to train young diplomats is named for him, is not the only prominent signatory of the report. Others include Lee H. Hamilton, who was a leader of the commission on the attacks of Sept. 11, 2001, and serves on Mr. Obama’s intelligence advisory board; Anne-Marie Slaughter, the director of policy planning during Mr. Obama’s first term; and Ryan C. Crocker, who served in many prominent ambassadorial positions. Former Senator Richard G. Lugar of Indiana, an early mentor to Mr. Obama in the Senate, was among the Republican signatories.

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