Talks between Russia and the US aimed at reaching agreement on accounting for and disposing of Syria's chemical weapons are to enter a third day.
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 officials say the talks in Geneva between Secretary of State John Kerry and Russian counterpart Sergei Lavrov are tough but are making progress.
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.
One told the BBC they are estimating the size of Syria's chemical stockpile.
They are thrashing out the technical details of the disarmament process.
A UN report expected next week will "overwhelmingly" confirm that chemical weapons were used in Syria last month.
UN Secretary General Ban Ki-moon says a UN report will "overwhelmingly" confirm that poison gas was used last month.
UN Secretary General Ban Ki-moon made no comment on who was to blame for the 21 August attack in the Ghouta area of Damascus, as that is not part of the report's remit.
He made no comment on who was to blame for the 21 August attack in eastern Damascus.
But he did say Syria's president was guilty of "crimes against humanity".
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.
A spokeswoman for Russian Foreign Minister Sergei Lavrov said the talks between the minister and Mr Kerry in Geneva were "on some real substance".
The BBC's Nick Bryant in New York says Mr Ban appeared not to have realised his comments were being broadcast.
'Accountability'
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.
Correspondents sat that the negotiators in Geneva still face considerable technical and political challenges when they resume.
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.
Earlier, the two men said they had agreed to discuss preparations for a peace conference on Syria for later this month.
A US official told AP news agency that the talks were at a "pivotal point".
Some progress has been made on how to account for Syria's chemical weapons inventory, the official said, adding that both the US and Russia have narrowed their differences over what each country believes to be the size of the Syrian stockpiles.
Washington and its allies accuse the Syrian government of killing hundreds of people in a chemical attack in Ghouta. The government denies the allegation, blaming rebels.
Mr Ban said the UN findings would be "an overwhelming report that chemical weapons were used".
Speaking on Friday, Mr Ban made no comment on blame but did say that Syrian President Bashar al-Assad had "carried out many crimes against humanity".
The BBC's Nick Bryant says Mr Ban appeared not to have realised his comments at the UN Women's International Forum were being broadcast.
But they were shown on UN television.
A senior UN diplomat told the BBC that Mr Ban would brief the Security Council on the report in New York at 11:00 local time (15:00 GMT) on Monday.
Diplomats said that while the report might not lay explicit blame, its factual reporting based on soil, blood and urine samples - and interviews with doctors and witnesses - could suggest who was responsible.
'Real negotiating'
The negotiations in Geneva are mainly concerned with a Russian plan to place Syria's chemical weapons under international control.
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.
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.
The plan led President Barack Obama to put off calls for the US Congress to vote on a campaign of military strikes.
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.
A Russian foreign ministry spokeswoman told Reuters news agency: "It is a sign that we are going on, that we proceed with talking and negotiating. Now it is like a real negotiating process, they are working on some real substance."
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.
Both Mr Kerry and Mr Lavrov met UN Arab League envoy Lakhdar Brahimi to discuss the potential for a broader peace conference in Geneva at a later.
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.
Mr Kerry had earlier described the talks as "constructive" and President Obama said he hoped they would "bear fruit", adding: "But I repeated what I've said publicly, which is any agreement needs to be verifiable and enforceable."
The Russians had objected to any threat of force, and to any resolution that would blame the Assad regime for the Damascus poison-gas attack.
Diplomacy will continue over the weekend, with Mr Kerry visiting Israel on Sunday.
White House officials have now briefed reporters that the US is willing to drop its position on the use of force.
Mr Lavrov said he welcomed the chance to discuss the "longer-term goal" of peace in Syria, and that now Syria had joined the Chemical Weapons Convention it was necessary "to design a road which would make sure that this issue is resolved quickly, professionally, as soon as is practical".
And the US side has now begun to talk only of unspecified consequences for non-compliance.
The BBC's James Robbins in Geneva says it appears the pair of envoys get on well, but that until they agree on the narrower issue of chemical weapons, reviving the wider peace talks feels like a distant prospect.
President Obama said earlier that "any agreement needs to be verifiable and enforceable".
If the Geneva talks are successful, the US hopes the disarmament process will be agreed in a UN Security Council resolution.
The White House officials also said the US would reserve the right to take military action without UN backing.
However, Russia regards as unacceptable any resolution backed by military force. The Obama administration for its part has signalled its willingness to compromise on a possible UN resolution punishing Syria.
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".
Senior White House officials are quoted as saying that President Obama will no longer demand that it includes the threat of force - the major sticking point with Russia.
Instead, they told reporters America would reserve the right to take military action without UN backing.
More than 100,000 people have died since the uprising against President Assad began in 2011. Millions of Syrians have been displaced.
More than 100,000 people have died since the uprising against President Assad began in 2011. Millions of Syrians have been displaced.
