French President Emmanuel Macron has warned Lebanese politicians they risk sanctions if they fail to set the nation on a new course within three months, stepping up pressure for reforms in a country collapsing under the weight of an economic crisis.

Visiting for the second time in less than a month, Mr Macron marked Lebanon's centenary by travelling to a forest outside Beirut to plant a cedar tree, the emblem of a nation facing the biggest threat to its stability since the 1975-1990 civil war.

"It's the last chance for this system," Mr Macron told Politico in an interview while travelling to Beirut yesterday.

"It's a risky bet I'm making, I am aware of it ... I am putting the only thing I have on the table: my political capital."

Mr Macron said he was seeking "credible commitments" and a "demanding follow-up mechanism" from Lebanon's leaders, including a legislative election in six to 12 months.

Should they fail to shift direction in the next three months, he said, punitive measures could be imposed, including withholding bailout money and sanctions on the ruling class.

Lebanese politicians, some of them former warlords who have overseen decades of industrial-scale corruption, face a daunting task with an economy in meltdown, a swathe of Beirut in tatters after the 4 August port explosion and sectarian tensions rising.

Beirut blast
The Beirut port explosion killed more than 190 people and injured 6,000

Mr Macron told online news provider Brut that he would visit Lebanon again in December.

Hours before he arrived yesterday, a new prime minister was designated, Mustapha Adib, following a consensus among major parties forged under pressure from Mr Macron over the weekend.

Mr Macron said he would use his weight to press for the formation of a new government. Without reforms, funds pledged at a 2018 donor conference in Paris would not be released, he said.

Mr Macron visited Beirut in the immediate aftermath of the port explosion that killed more than 190 people and injured 6,000.

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He said the international community must stay focused on the emergency in Lebanon for six weeks and that he was ready to help organise an international conference, in coordination with the United Nations, in mid to late October.

"I am ready to host it in Paris," he said.

Mr Macron has taken centre stage in demanding change, but other foreign powers still exercise big influence on Lebanon, notably Iran through the heavily armed Shi'ite group Hezbollah.

A senior envoy from the United States, which classifies Hezbollah a terrorist organisation and which has pumped money into the Lebanon's army, is due in Beirut tomorrow.

Saudi Arabia also has traditionally exercised sway through the Lebanon's Sunni community.