The panel’s findings further damaged the credibility of President Bashar al-Assad of Syria, who, under international pressure, signed a treaty banning chemical weapons nearly three years ago after a horrific attack in which the nerve agent sarin killed hundreds in a Damascus suburb.

The United States has accused Mr. Assad’s forces of responsibility for that attack. Mr. Assad and his subordinates have consistently denied government forces have used chemical weapons in the conflict.

Mr. Assad’s compliance with commitments made by signing the chemical weapons treaty have long been suspect. Although all of Syria’s declared stockpile of dangerous ingredients to make chemical weapons was exported and destroyed, that operation took far longer than expected and raised questions about whether all had been accounted for. The director general of the Organization for the Prohibition of Chemical Weapons has complained in an internal report about misleading statements from Syria and potentially undeclared “chemical-weapons-related activities” there, Foreign Policy magazine reported on Tuesday, citing what it described as a confidential two-page summary of the report.

The organization’s public affairs office declined to comment.

There was no immediate comment from the Syrian government on the findings in the report submitted to the Security Council on Wednesday. But they could set up a new confrontation at the Security Council between Russia and the United States over whether to impose new penalties on Syria.

Image A civilian breathes through an oxygen mask after a hospital and a civil defense group said a gas, which they believed to have been chlorine, was dropped alongside barrel bombs on a neighborhood in Aleppo, Syria, on Aug. 11. Credit... Abdalrhman Ismail/Reuters

The Russians, Mr. Assad’s most important ally, have blocked previous moves by the Security Council to penalize Mr. Assad for brutalities committed since the conflict in Syria began more than five years ago.