The United Nation's secretary-general condemned on Monday an August 21 chemical weapons attack near Damascus, calling it a "war crime." Ban Ki-moon made his remarks during a press conference following the presentation of a UN inspectors report on the strike, which killed hundreds of Syrian civilians.

The report said there is "clear and convincing evidence" that chemical weapons were used on a relatively large scale in an attack last month.  Ban presented the UN inspectors' report to a closed meeting of the UN Security Council before speaking to the press.

"I trust all can join me in condemning this despicable crime. The international community has a responsibility to hold the perpetrators accountable," Ban said.

"Our message: Do not slaughter your people with gas," the secretary-general warned. "Those who do will face justice.

"This is the most significant confirmed use of chemical weapons against civilians since Saddam Hussein used them in Halabja in 1988," he added.

Still, Ban declined to assign blame, saying it was for "others to decide" who was behind the attack.

However, the US ambassador to the UN, Samantha Power, accused Syrian President Bashar Assad directly, telling reporters that "the technical details make it clear that only the regime could have carried out this large-scale attack."

Earlier in the day, the top diplomats from the United States, France and Britain stood side by side to pressure Assad to uphold his end of a deal on securing and destroying Syria's chemical weapons. France and the US insisted that a military response to the attack remains on the table, and pressed for a robust UN resolution.

The United Nations Security Council is set in the coming days to take up the resolution, laying out plans for the agreement brokered by the United States and Russia.

"If Assad fails to comply… we are all agreed, and that includes Russia, that there will be consequences," US Secretary of State John Kerry said ahead of meetings in Paris with his counterparts from Saudi Arabia and Turkey, two of the strongest proponents of military action against Assad.

"It is extremely important that there are no evasions, that there is no cat-and-mouse game going on over these weapons," said British Foreign Secretary William Hague.

Meanwhile, invitations went out Monday to top members of the Syrian National Coalition — the main umbrella opposition group — for an international conference in New York timed to coincide with next week's UN General Assembly meeting, French officials said.

Bolstering the Western-backed SNC is just as crucial to Syria's future as Assad's agreement to give up chemical arms, explained French Foreign Minister Laurent Fabius.

"He must understand that there is no military victory, no possible military victory for him," Fabius emphasized. He acknowledged that broad popular support for the rebels has been hampered by fears that Islamic militants are now playing a major role in the 2½-year-old uprising.

The August 21 attack was carried out as a UN chemical weapons team was in Syria to investigate earlier alleged chemical attacks. After days of delays, the inspectors were granted access to victims, doctors and others in the Damascus suburbs afflicted by the poison gas.

The Assad regime continues to insist that the attack was carried out by rebels.

Asher Zeiger and AP contributed to this report.