UN experts will give "overwhelming" confirmation that chemical weapons were used in Syria, UN leader Ban Ki-moon predicted Friday, as he blasted President Bashar al-Assad as a war criminal.

Ban did not say that Assad's forces carried out a suspected chemical arms attack near Damascus investigated by UN experts. But he said the Syrian leader has "committed many crimes against humanity."

A UN team is expected to send its report on the August 21 attack to Ban on Monday. The UN leader stressed that he still does not have the report.

But he predicted: "I believe the report will be an overwhelming report that the chemical weapons were used."

Ban also gave a UN estimate that 1,400 people were killed in the August 21 attack at Ghouta, east of Damascus, which led to western threats of a military strike on Assad's forces.

UN inspectors were in Syria to investigate the general use of chemical weapons in the country's 30-month-old conflict when the suspected sarin gas attack took place.

The United States, Britain and France blame Assad for the attack. The Syrian government, backed by Russia, say opposition rebels used the banned gas.

UN investigation leader Ake Sellstrom is expected to send his report to Ban on Monday. Sellstrom is not allowed to say in the report who carried out the attack.

Diplomats say however that details, such as the type of chemical and missile involved, could indicate who carried out the attack.

If the report is delivered Monday, Ban is expected to brief the UN Security Council within hours on its findings.

While US Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov are discussing a plan to put Syria's chemical arms under international control, the Security Council's permanent members are discussing a possible resolution on the crisis.

Ban has stepped up criticism of the Security Council for its failure to act over Syria. But he has never hidden his disdain for the tactics used by the Syrian leader in a civil war in which well over 100,000 people have died.

In his new outspoken attack, Ban said that Assad must face "accountability" as part of any political solution to end the conflict.

"What happened is that he has committed many crimes against humanity. Therefore, I'm sure that there will be surely the process of accountability when everything is over," he said.

Ban said last year that Assad had "lost all humanity".

His latest comments came after a UN inquiry said Wednesday that government forces had committed "widespread" killings and torture of civilians.

But Ban stressed that the top priority must be "to help the fighting stop and dialogue, talking begin."

In parallel to Ban's comments, the UN humanitarian chief raised concerns about access to areas around Damascus where hundreds of thousands of people are trapped by the fighting.

"People are unable to leave sealed-off government or opposition-held areas, sometimes for months on end, and have run short of water, food, power and medicines," said Valerie Amos.

"I am extremely worried by reports that more than 500,000 people remain trapped in rural Damascus."

Amos said there were "very disturbing reports" about Moadamiyet al-Sham, near Damascus.

"It is reported that the town has been besieged for the last 10 months, suffering daily shelling and armed clashes between government and opposition groups," the head of the UN Office for Coordination of Humanitarian Affairs said.

Amos said UN agencies have been unable to get supplies into the town for nearly a year. Most of the 70,000 people in the area have fled, but some 12,000 people remain trapped and there are cases of severe malnutrition among children, the UN said.

Amos said the UN has taken $50 million from its emergency finances to bolster the depleted fund to get supplies to people inside Syria and help the more than two million people who have fled to neighboring countries.