French PM Jean-Marc Ayrault has presented intelligence to MPs which he says shows Syria used chemical weapons, reports BBC News.
The dossier is said to show that Syria has large chemical stockpiles and was behind a chemical attack which is thought to have killed hundreds.
France and the US are both pushing for punitive military action against the Syrian regime. The UK parliament has voted to stay out of such a raid.
Syrian President Bashar al-Assad has again denied being behind the attack.
In an interview with French newspaper Le Figaro, to be released on Tuesday, he said it would have been "illogical".
He also warned that foreign military action could ignite a wider regional conflict.
"Everyone will lose control of the situation when the powder keg explodes. Chaos and extremism will spread", he said.
Mr Assad also warned France that there would be "repercussions" to any involvement in Syria.
The alleged chemical attack took place in the Ghouta, an eastern area of the capital on 21 August. The US says 426 children were among the more than 1,400 people killed.
The US administration has already presented its case that the Assad regime was behind the attack, and now Mr Ayrault is presenting France's own intelligence dossier to parliamentary leaders.
"We are going to give the MPs everything we have - classified until now - to enable every one of them to take on board the reality of the unacceptable attack," he said on Monday.
French MPs are due to debate the issue at an extraordinary session of parliament on Wednesday.
President Francois Hollande is constitutionally able to order an attack without parliamentary approval, but there is growing pressure for a vote on Syria, as happened in the UK and has been promised in the US.
US lawmakers are due to reconvene next week, and White House officials have said that when it comes to a vote, they believe there will be enough support for the president.
US Secretary of State John Kerry said on Sunday samples from hair and blood gathered after the 21 August attack had tested positive for "signatures of sarin", and that he was confident Congress would give its approval for strikes, "because they understand the stakes".
However some lawmakers have expressed doubts about President Barack Obama's plan for a "limited, narrow" military operation, questioning its purpose and effectiveness.
Read more of this report from BBC News.