President Obama addresses the 68th session of the United Nations General Assembly at the United Nations headquarters on Tuesday. (Pablo Martinez Monsivais/AP)

The United States and Russia traded accusations Tuesday about who was to blame for the nerve gas massacre in Syria last month and about who is now to blame for a diplomatic impasse at the United Nations.

President Obama ridiculed the idea — voiced most loudly by Russia — that the Syrian government was not behind the Aug. 21 attack that killed about 1,400 people.

The evidence that the Russian-backed regime of Syrian President Bashar al-Assad carried out the attack “is overwhelming,” Obama said in his annual address to the U.N. General Assembly. “It’s an insult to human reason and to the legitimacy of this institution to suggest that anyone other than the regime carried out this attack.”

A U.N. team is returning to Syria on Wednesday to look more deeply into the use of chemical weapons there, Russian and U.S. diplomats said. The weapons inspectors plan to finish an investigation of alleged chemical weapons incidents that was partly sidelined by the much larger attack last month.

Russia, for its part, stuck to its position that the large-scale use of chemical weapons was more likely the work of rebel forces, and it accused the United States of blocking progress at the United Nations.

“Even though they agreed to a compromise on chemical weapons, U.S. representatives keep insisting that the Syrian regime, as they call it, is responsible,” Russian Deputy Foreign Minister Sergei Ryabkov said Tuesday. “Without presenting any irrefutable evidence, they are saying all the time that the plans to ‘punish’ Damascus, even through military intervention, remain in force.”

The cooperation and disagreements between the United States and Russia over Syria hung over Obama’s two-day visit to the world body. The two nations agreed Sept. 14 on a plan to seize and destroy Syrian chemical weapons stores, averting an imminent U.S. attack on Syrian military sites, but are at odds over how to enforce the initiative at the U.N. Security Council.

Obama drew attention to the stalled resolution that would authorize some manner of penalty for Syria if it refuses to give up its chemical weapons. “The Syrian government took a first step by giving an accounting of its stockpiles,” he said. “Now, there must be a strong Security Council resolution to verify that the Assad regime is keeping its commitments, and there must be consequences if they fail to do so.”

Russia has been opposed to any action against Syria, though it has suggested it might change its mind if it found that the Syrian government was indeed guilty of using chemical weapons.

“Regrettably,” Ryabkov said, “I have to admit that our contacts with Americans are not very smooth and, generally speaking, are not moving in the direction in which they should be moving today.”

Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov met for about 90 minutes later Tuesday to try to bridge differences over the U.N. resolution. U.S. officials described the session as very productive and substantive, with Kerry and Lavrov marking up a draft text. But they would not say whether the key division over how to enforce the chemical weapons deal was resolved. The United States wants the resolution to fall under the part of the U.N. mandate that could allow military force, even though none is planned.

A senior State Department official, who spoke on the condition of anonymity to give some detail about the closed-door session, said the U.S. goal is to eliminate “any ambiguity” about the intent to rid Syria of its chemical weapons and the potential for consequences if the Assad regime balks.

Officials would not say whether they expect the resolution to be ready for a Security Council debate and vote this week.

Obama made a point Tuesday of telling U.N. members that it was only because of the possibility of a U.S. attack that an agreement to dismantle Syria’s chemical arsenal emerged. That threat remains if diplomacy fails, Obama said. He added that if the United Nations fails to act, it will show itself unable to enforce a basic international rubric against chemical warfare.

The U.N. report on the Aug. 21 attack did not assign blame, but Western governments have said the evidence makes it clear the sarin came from forces under Assad’s direction.

“Our position is that this report requires serious and scrupulous analysis,” Ryabkov said, “while the conclusions that were publicly announced as soon as the report was presented were too hasty.”

Kerry also met Tuesday with Syrian opposition leader Ahmad al-Jarba, whose U.S.-backed movement has complained that the U.S.-Russian deal sidelines them. The rebels had hoped to benefit from the limited U.S. strikes Obama contemplated, even though he had insisted that strikes would not be the start of any wider U.S. military intervention on the rebels’ behalf.

“They certainly did express disappointment that there hadn’t been a military strike,” another U.S. official said after that meeting. That official also spoke on the condition of anonymity to discuss the sensitive diplomacy.

Kathy Lally in Moscow contributed to this report.