Saudi Arabia and France sealed an agreement Tuesday for Riyadh to finance the delivery of $3 billion worth of French weapons to the Lebanese army, which has come under mounting jihadist attack.

French Foreign Minister Laurent Fabius hailed the conclusion of the deal, first announced last December, as a major boost to the Lebanese army's ability to tackle "terrorism" at a time when the former French colony is under mounting threat.

The deal comes as the poorly equipped Lebanese army battles jihadists, including militants of the Islamic State group, both along its porous border with Syria and in its second city Tripoli.

The deal was signed in Riyadh by Saudi Finance Minister Ibrahim al-Assaf and Edouard Guillaud, the head of the ODAS organisation set up by France for the export of defence equipment, a diplomat told AFP.

Lebanese army chief General Jean Kahwaji was also present at the ceremony, the diplomat said, without giving details on the list of weapons to be supplied -- a clause that had stalled the agreement for months.

A French source told AFP that the contract would now "be rapidly implemented."

The French foreign minister said: "This agreement, financed through Saudi aid, will contribute to strengthening the Lebanese army, guarantor of Lebanon's unity and stability.

"It will help it to carry out its mission to defend national territory and fight terrorism, at a time when Lebanon is under threat."

Lebanon's main northern city of Tripoli was rocked by three days of devastating fighting between troops and suspected Al-Qaeda loyalists late last month that left at least 11 soldiers and five civilians dead.

In August, troops fought deadly clashes with jihadists of the Islamic State group and Al-Qaeda affiliate Al-Nusra Front in the town of Arsal close to the Syrian border.

The jihadists withdrew across the border after a truce deal, but took with them several dozen captive Lebanese soldiers and police, three of whom they have since executed.

Last December, OPEC kingpin Saudi Arabia agreed to finance a $3 billion package of French military equipment and arms for the Lebanese army.

And in mid-June, at a conference in Rome, the international community pledged its backing for the Lebanese military.

But in September, Lebanese Foreign Minister Gebran Bassil said his country was still waiting "impatiently" for the delivery of the French-made weapons.

In August, Saudi Arabia pledged a further $1 billion to strengthen the Lebanese army and last month Washington announced it had delivered a new shipment of Hellfire missiles and would also supply light aircraft.

Announcing the supplies, US ambassador David Hale said the aircraft would be paid for out of the additional Saudi funding.