French Prime Minister Jean-Marc Ayrault is presenting intelligence to MPs which he says shows Syria used chemical weapons.
The dossier is said to show that Syria has large chemical stockpiles and was behind a chemical attack which the US says killed more than 1,400 people.
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.
Damascus denies the attack, blaming rebel forces for the use of chemicals.
However, NATO Secretary General Anders Fogh Rasmussen has said he is personally convinced that a chemical attack took place and that the government of President Bashar al-Assad was responsible.
There must be “a firm international response” to deter any future use of such weapons, he said, or else it would send a “dangerous signal to dictators all over the world”.
But he added that he did envisage NATO having a role in such action, saying he would expect any military response to be “a very short, measured, targeted operation” and that the alliance’s resources would not be needed.
Meanwhile fighting has continued across Syria, in a conflict which has already left an estimated 100,000 people dead since 2011. On Monday, activists said 20 rebel fighters were killed in an army ambush in Adra, north-east of Damascus, AFP news agency reports.
The alleged chemical attack took place in the Gouta, 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 Jean-Marc Ayrault will present 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.
Secretary of State John Kerry said on Sunday samples from hair and blood gathered after the August 21 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.
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