French Prime Minister Jean-Marc Ayrault

France unveiled intelligence reports asserting the Syrian regime’s responsibility for a “massive and co-ordinated” chemical attack in Damascus earlier this month as the socialist government sought to rally support for France’s commitment to join a potential retaliatory military strike.

President François Hollande has faced mounting demands to justify his stance following the surprise decision by President Barack Obama to seek congressional approval for any military action, with some opposition leaders warning him against becoming “towed along” by the US.

In a move to fend off growing demands for a parliamentary vote, Prime Minister Jean-Marc Ayrault briefed party leaders ahead of a debate on Wednesday, showing them intelligence that the government said showed the regime of President Bashar al-Assad carried out the attack in a Damascus suburb on August 21.

Mr Ayrault afterwards reiterated France’s determination “to sanction the regime of the dictator Bashar al-Assad”, holding to the commitment to “firm and proportional” action despite Mr Obama’s move, although repeating that France would not act alone.

The intelligence dossier asserted that the attacks came from government-controlled areas and targeted rebel-held zones. It said opposition forces did not have the means to carry out such operations.

The document said credible intelligence showed the tactical attack had been prepared several days before and had the aim of retaking territory, unlike earlier and smaller alleged strikes that used chemical weapons against rebels. It was carried out using rockets. The operation was followed the next day by artillery assaults in an attempt to destroy the evidence.

It said video analysis showed at least 281 deaths – but said other estimates of about 1,500 killed were consistent with French modelling of the effects of such an attack. The US said more than 1,400 people died.

In an interview with Le Figaro newspaper, President Assad dismissed the allegations, saying Mr Obama and Mr Hollande were “incapable” of providing proof. He said there would be “negative repercussions” for France if it joined an attack on Syria.

On Monday, Alain Juppé, former prime minister and foreign minister under former president Nicolas Sarkozy, joined senior opposition figures insisting that Mr Hollande, like Mr Obama and UK prime minister David Cameron, hold a parliamentary vote to approve military intervention.

“Even though the constitution does not require it, I think in the current circumstances parliament should pass a vote so the president is supported at least by the backing of the national representation,” said Mr Juppé, who backs action against the Damascus regime.

Mr Hollande has so far refused to agree to a vote.

The British vote against military intervention and Mr Obama’s move unleashed a political debate in France over Mr Hollande’s determination to commit French forces to a limited operation to “punish” Damascus.

The Elysée Palace remains confident that the president has majority parliamentary support, despite concerns voiced by those such as François Fillon, prime minister under Mr Sarkozy, about taking action without UN Security Council authority.

Mr Fillon and others in the centre-right UMP have also warned Mr Hollande against becoming hostage to Washington and the vote in Congress. The government in Paris has acknowledged it cannot act militarily without the US.

Mr Juppé called on Mr Hollande to make an address to the nation. “The situation is serious,” he said. “[The president] should speak to the French people to give them all the necessary information and explain the detail of his strategy.”

There is opposition to military action on the far left and the far-right as well as within the mainstream UMP. However, Mr Hollande’s position is aided by a lack of UMP unity. Jean-Francois Copé, UMP president, has not insisted on a parliamentary vote and signalled his qualified support for a military strike.

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