"I believe there was a chemical attack and we are looking for the actual evidence", he said, indicating that a decision is yet to be made.
The same officials say Syria has continued to produce or procure chlorine, which also has industrial and agricultural uses.
Asked about possible USA strikes on Syria, President Vladimir Putin's spokesman Dmitry Peskov said Thursday that "it's necessary to avoid any steps that may fuel tensions in Syria".
The White House and National Security Council did not immediately respond to Anadolu Agency's requests for comment on the report.
"We have not yet made any decision to launch military attacks into Syria", Mattis told lawmakers on the House Armed Services committee.
Russia's United Nations ambassador warned the priority in Syria was to avert US-led strikes that could lead to a confrontation between the world's two preeminent nuclear powers. That meant airstrikes, possibly in tandem with France and other allies that have expressed outrage at the alleged Syrian chemical attack, could be launched within hours of a presidential decision. A joint military operation, possibly with France rather than the U.S.in the lead, could send a message of global unity about enforcing the prohibitions on chemical weapons.
Geopolitics specialist Dominique Moisi, a senior adviser at the Montaigne Institute?think tank in Paris, said "we have said that we were not allowing the use of chemical weapons, that this was a red line ..." Trump's ambassador to the United Nations, Nikki Haley, told NBC News on Thursday the administration has "enough proof" of the chemical attack but is still considering its response. A military operation would be a big deal for Macron, his first military action as president, at a time of heightened domestic tensions over labor law changes. Asked if France is planning to participate in retaliatory attacks on Syria, he was noncommittal.
"So striking at Syria is not a good solution, but doing nothing after the use of chemical weapons is even worse", he said.
Peskov wouldn't say if Moscow could use a Russia-U.S. military hotline to avoid casualties in a case of a U.S. blow, saying only that "the hotline exists and has remained active". More broadly, he doubted the wisdom of bombing. "We need to know where that's going, what the goal of it is before we take that act".
US Ambassador Nikki Haley said no decision had been taken to launch strikes and stressed Washington was taking time to assess the implications of possible military action after Russia warned it could lead to a unsafe US-Russian war.
At the House hearing, Rep. Tulsi Gabbard, D-Hawaii, disputed Trump's legal authority to act without congressional authority and suggested a US strike would lead to war with Russian Federation.