Macron said France's information had shown "that chemical weapons were indeed used and that the regime could clearly be held responsible".
Asked whether France would take military action, Macron said his country will continue discussing technical and strategic information with USA and British allies and "in the coming days we will announce our decision".
Despite rising threats of war, Macron stressed the necessity to avoid "an escalation" in the region, which has already suffered long and bloody civil wars that have forced millions to displace and seek refuge in Europe.
A team of experts from the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, was travelling to Syria and will start their investigations on Saturday, the Netherlands-based agency said.
France is expected to join the United States (US) and Britain in carrying out air strikes or some other form of attack in response to the use of the weapons but it remains unclear when that might happen or even if it definitely will. "Could be very soon or not so soon at all", he added.
US President Trump, however, warned that Russia, Iran and Syria will have a "big price to pay" following the suspected chemical attack, saying his administration will soon be making "some major decisions" on Syria. The finance ministry says it hopes the program, the improved deficit targets as well as the economic reforms led by the government will help France exit the excessive deficit procedure of the EU Commission.
Macron said he would decide "in the coming days" on his response, in conjunction with the U.S. and Britain.
Senior Russian figures have warned that missiles threatening the country's forces in Syria will be shot down and their launch sites targeted.
Meanwhile, Trump tweeted a clarification saying he had never said when the United States would attack Syria.
Turkish President Recep Tayyip Erdogan says he will discuss ways of ending "the chemical massacre" in Syria during a telephone call with Russia's President Vladimir Putin.
However, after persistent reports of chlorine attacks over the previous year, his foreign minister and aides have been more nuanced saying a response would hinge on French intelligence proving both the use of chemicals and fatalities, and a riposte would most likely be in coordination with the US.
More than 13,500 Syrian rebel fighters and their families have left Douma this month under a so-called evacuation deal between the rebels and the Russian military, a top ally of Syrian President Bashar Assad's government.