PARIS - France's foreign minister said on Thursday the Security Council had to respond firmly after a UN probe found that Syrian government troops were responsible for two toxic gas attacks."The use of chemical weapons, which the Syrian authorities agreed to ban in 2013 under international pressure ... is an abomination that highlights the damning role played by the Damascus regime in the worsening situation in Syria," Foreign Minister Jean-Marc Ayrault said in a statement.A joint investigation by the United Nations and the global chemical weapons watchdog also found that Islamic State militants had used sulfur mustard gas, according to a confidential report seen by Reuters."France calls for a response that measures the seriousness of this. The Security Council must not shirk its responsibilities and France is already working with its partners" on the matter, Ayrault said.The results set the stage for a Security Council showdown between the five veto-wielding powers, likely pitting Russia and China against the United States, Britain and France over whether sanctions should be imposed in the wake of the inquiry.French diplomatic sources said Paris wanted to use the chemical weapons report to push for a resolution at the Security Council that would force Russia, the key backer to Syrian President Bashar al-Assad, to accept that its ally had used banned weapons and as a result would use its influence to back a credible ceasefire across the country."We need to use this report to force Russia out of its ambiguity. If they were to veto such a resolution they would need to justify the use of chemical weapons," one senior diplomat said.The 15-member Security Council is due to discuss the report next week. UN Secretary-General Ban Ki-moon said the report would be made public after that meeting.