The US will drop its insistence that a UN resolution on Syria must be backed by military force, officials say, after strong objections from Russia.
US and Russian diplomats say the two sides are edging closer to a deal on Syria’s chemical arsenal, as talks in Geneva enter a third day.
They are thrashing out the technical details of the disarmament process.
UN Secretary General Ban Ki-moon says a UN report will “overwhelmingly” confirm that poison gas was used last month.
He made no comment on who was to blame for the 21 August attack in eastern Damascus.
But he said Syrian President Bashar al-Assad had committed “many crimes against humanity”, in comments at the UN Women’s International Forum that were shown on UN television.
The BBC’s Nick Bryant in New York says Mr Ban appeared not to have realised his comments were being broadcast.
The US had threatened military action against Mr Assad’s regime, accusing the military of killing more than 1,400 people in the 21 August attack.
But Mr Obama called off a Congress vote on the strikes after Russia announced a plan to place Syria’s chemical weapons under international control and have them destroyed.
Syria has agreed to the plan and has sent documents to the UN to sign up to the Chemical Weapons Convention, which outlaws the production and use of the weapons.
US Secretary of State John Kerry is meeting his Russian counterpart Sergei Lavrov in Geneva to thrash out the technical details of seizing and destroying the chemical stockpile.
Officials said some progress had been made on how to account for Syria’s chemical weapons inventory, with the US and Russia narrowing their differences over estimates of the size of the stockpiles.
But a major point of contention is reported to be the US insistence that any deal on chemical weapons be enshrined in a UN resolution, backed by the threat of military force for non-compliance.
White House officials have now briefed reporters that the US is willing to drop its position on the use of force.
And the US side has now begun to talk only of unspecified consequences for non-compliance.
President Obama said earlier that “any agreement needs to be verifiable and enforceable”.
The White House officials also said the US would reserve the right to take military action without UN backing.
Mr Lavrov said it was necessary “to design a road which would make sure that [the chemical weapons] issue is resolved quickly, professionally, as soon as is practical”.
More than 100,000 people have died since the uprising against President Assad began in 2011. Millions of Syrians have been displaced.