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.
US officials say the talks in Geneva between Secretary of State John Kerry and Russian counterpart Sergei Lavrov are tough but are making progress.
One told the BBC they are estimating the size of Syria’s chemical stockpile.
A UN report expected next week will “overwhelmingly” confirm that chemical weapons were used in Syria 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.
But he did say Syria’s president was guilty of “crimes against humanity”.
Ban Ki-moon: “He (Assad) has committed many crimes against humanity”
A spokeswoman for Russian Foreign Minister Sergei Lavrov said the talks between the minister and Mr Kerry in Geneva were “on some real substance”.
Correspondents sat that the negotiators in Geneva still face considerable technical and political challenges when they resume.
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.
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.
The plan led President Barack Obama to put off calls for the US Congress to vote on a campaign of military strikes.
A Russian foreign ministry spokeswoman told Reuters: “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.”
One US official told the agency the talks were at a “pivotal point”.
Mr Kerry had earlier described the talks as “constructive” and President Obama said after meeting Kuwait’s emir at the White House that he hoped they would “bear fruit”.
“But I repeated what I’ve said publicly, which is any agreement needs to be verifiable and enforceable,” Mr Obama said.
Diplomacy will continue over the weekend, with Mr Kerry visiting Israel on Sunday.
UK Foreign Secretary William Hague and French counterpart Laurent Fabius will meet Mr Kerry in Paris on Monday.
He said both the US and Russia were “deeply committed to a negotiated solution” to the conflict in Syria and he and Mr Lavrov were “working hard to find the common ground to be able to make that happen”.
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”.
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.
If the Geneva talks are successful, the US hopes the disarmament process will be agreed in a UN Security Council resolution.
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.
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.