France on Syria: We are not being led by US

He noted: "We're not simply following … We're ready to take this decision to stop [alleged chemical weapons use]. We can't do it alone. We've wanted an international coalition from the start, not just militarily but politically."

He said France was the first to recognise the opposition group, the Syrian National Council, as the legitimate representative of Syria.

He added: "Our position is clear. It's the US President's right to call Congress. But we're not following a US decision. It's our own decision that we can finally put in place, with a need to be clear, firm and fast."

He also denied claims by the centre-right UMP party that France and the US are isolated in the UN and in Europe.

"France is not acting alone … We are counting on the support of the Europeans and of countries in the region, notably those in the Arab League," he said.

Ayrault pledged that operations will not involve ground troops and will not be designed to create regime change.

But he noted: "Of course, we want the departure of [Syrian leader] Bashar al-Assad, who has not hesitated to directly threaten France, our country."

For his part, the UMP leader, Christian Jacob, said his side would not back any operation which does not have UN blessing.

He also called for a vote in the French parliament, on the model of a recent British vote which overturned the UK government's plans for joint US strikes.

"We reject an isolated action without international legitimacy," Jacob said.

"You cannot escape a vote," he added.

The minor parties in the French assembly echoed Jacob's scepticism.

The centrist UDI called for a UN mandate and cast doubt on the quality of French intelligence that al-Assad launched a large-scale gas attack against civilians last month.

The left-wing FDG and RPG parties said that only a political solution can bring peace, while the far-right Front national said jihadist groups will control Syria if al-Assad falls.

The green EELV faction voiced internal division, however, while the former UMP prime minister, Alain Juppe, came out against his own party on French TV on Thursday morning, saying that waiting for the UN, where Syria's ally, Russia, has a veto, amounts to "collusion on inaction."

French President Francois Hollande is to meet with leaders from EU countries in the G20 club - Germany, Italy and the UK - in the margins of a summit in St Petersburg on Thursday to discuss a joint European position.

Germany has said its forces will not take part in operations.

But on Wednesday it continued to build political backing for action, when its intelligence service, the BND, briefed MPs in a confidential meeting in Berlin on evidence that al-Assad did it.

"The BND referred to a phone call they had heard between a Hezbollah official and the Iranian embassy in which he spoke about al-Assad having ordered the attack," one German deputy told Reuters, referring to a pro-Assad Lebanese group.

Italy has said any strikes must have a UN mandate.

Meanwhile, Swedish leader Fredrik Reinfeldt sat on the fence during a press conference in Stockholm on Wednesday with US leader Barack Obama.

"Sweden believes that serious matters concerning international peace and security should be handled by the United Nations. But I also understand the potential consequences of letting a violation like this go unanswered," Reinfeldt said.

The big confrontation at the G20 event is set to be between Obama and Russian President Vladimir Putin, who has repeatedly warned the West against getting involved.

He told press on Tuesday that if the US strikes Syria "we [Russia] shall think how we should act in the future, in particular regarding supplies of … sensitive weapons to certain regions of the world," alluding to deliveries of Russian anti-air defences to Iran, Israel's main adversary.