US Defense Secretary Jim Mattis Monday declined to rule out military airstrikes against Syria in response to its government’s alleged use of toxic gas against civilians in Douma. US administration sources said a first analysis suggests that nerve agent was used in the Syrian attack “but more evidence was needed.” Ahead of a meeting at the Pentagon with the emir of Qatar, Mattis said the first thing the administration is considering is why chemical weapons were used at all. The US would be consulting on this with allies in Europe, the Middle East and elsewhere. The defense chief noted that Russia was a guarantor of a deal with the Obama administration for the removal of all of Syria’s chemical weapons. Russian Foreign Minister Sergei Lavrov earlier called the allegation a “provocation” and asserted that Russian experts had failed to discover any evidence of chemical weapons.