While recognizing the argument for acting to prevent further use of chemical weapons, Ban said, "We must consider the impact of any punitive measure on efforts to prevent further bloodshed and facilitate a political resolution of the conflict."
"Our common humanity compels us to ensure that chemical weapons do not become a tool of war or terror in the 21st century," Ban told reporters before departing for Europe. "Any perpetrators must be brought to justice. There should be no impunity."
Ban called on the U.N. Security Council to develop an appropriate response if the allegation of chemical weapons use proves true.
"The Security Council has a duty to move beyond the current stalemate and show leadership," Ban said. "This is a larger issue than the conflict in Syria; this is about our collective responsibility to humankind."
Ban also warned that nations considering a military response that such a response is legal only in self-defense under the U.N. charter or if approved by the U.N. Security Council.
Syrian allies Russia and China have exercised their veto to block the Security Council from taking action against Syrian President Bashar Assad's regime.
Ban said he would use the Group of 20 summit in St. Petersburg, Russia, to "engage with world leaders on this tragedy, including humanitarian assistance."
"It is imperative to end this war," Ban said.
He said U.N. chemical weapons inspectors have worked "around the clock" since returning from Syria to prepare materials collected in Ghouta for analysis. All biomedical and environmental samples will be at their designated laboratories by Wednesday, Ban said.
"We are doing our utmost to expedite the process," he said.
Israel and the United States test-fired an "experimental target missile" Tuesday in the Mediterranean Sea, the Israeli Defense Ministry said.
The announcement from Israel came after Russia said its early warning system detected the launch of two ballistic missiles.
"The Ministry of Defense together with the American Agency for Missile Defense carried out the successful test of the experimental target missile Anchor," the Israeli ministry posted on its Facebook page.
The Defense Ministry said the launch took place at an Israeli Air Force base in the central part of the country and involved the firing and tracking of a Sparrow-type target missile, The Jerusalem Post reported.
In Damascus, Assad warned an attack on Syria could set off a huge conflict and dared the West to provide "the slightest proof" he used chemical weapons.
"The Middle East is a powder keg and the fire is approaching," Assad told French newspaper Le Figaro in an interview published Tuesday.
"Everyone will lose control of the situation once the powder keg explodes," he said. "Chaos and extremism will spread. The risk of regional war exists."
Assad accused U.S. President Barack Obama of being a weak leader.
"If Obama was strong, he would have said publicly, 'We have no evidence of the use of chemical weapons by the Syrian state.' He would have said publicly: 'The only way to proceed is through U.N. investigations. We therefore refer everything to the Security Council.' But Obama is weak because he is facing pressure from within the United States," Assad said.
The Syrian leader challenged Obama and French President Francois Hollande, a strong supporter of U.S. calls for a limited military attack, to provide bona fide, indisputable evidence to support their allegation Syrian forces killed what the Obama and Hollande administrations said were more than 1,400 civilians in a gas attack against rebel-held Damascus suburbs Aug. 21.
The Syrian regime denies the allegations and accuses rebels of releasing the gas hoping to draw foreign intervention against Assad. The opposition denies unleashing toxic substances.
France released a nine-page declassified document Monday evening it says proves "undeniably" Assad's forces carried out the "massive and coordinated" chemical attack.
The report, in allegations similar to those made by U.S. Secretary of State John Kerry, accused the regime of using "extremely lethal" chemical weapons -- including the deadly nerve agent sarin -- to spread "terror" among civilian populations.
"The past events and the simultaneous and massive use of chemical weapons on the night of 21 August 2013 in the eastern suburb of Damascus thus confirm that the Syrian regime has deliberately crossed a line," the report said, adding its intelligence indicated "other actions of this nature could still be carried out."
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