Leaders from Lebanon, Israel and the United States have all hailed the deal as "historic" but the possibility of a wider diplomatic breakthrough remains slim.

As a result, there was no joint signing ceremony: Lebanese President Michel Aoun signed a letter approving the deal at his palace in Baabda in the presence of the U.S. official who mediated the accord, Amos Hochstein.

"We have heard about the Abraham Accords. Today there is a new era. It could be the Amos Hochstein accord," said top Lebanese negotiator and deputy parliament speaker Elias Bou Saab, referring to the 2020 U.S.-brokered normalisation of ties between Israel and the United Arab Emirates and Bahrain.

Prime Minister Yair Lapid signed separately in Jerusalem, saying the deal was a "tremendous achievement" that had produced Lebanon's de facto recognition of Israel.

"It is not every day that an enemy country recognises the state of Israel, in a written agreement, in view of the international community," Lapid told his cabinet in broadcast remarks.

But Aoun later said the deal was purely "technical" and would have "no political dimensions or impacts that contradict Lebanon's foreign policy".

Lower-level delegations from each country headed to the United Nations' peacekeeping base at Naqoura along their contested land border, which has yet to be delineated.

POLITICAL MILESTONES

There, they separately submitted their signed copies of the deal to U.S. officials and their new coordinates for the maritime border to the U.N., officially bringing the deal into force.

The accord comes days ahead of major political milestones for both Israel and Lebanon.

Aoun's term is set to end on Oct. 31 and political sources say he was keen to seal the deal as the crowning achievement of his six years in office.

Israel holds elections on Nov. 1, its fifth in less than four years.

Hochstein said the accord should be adhered to even if officials change on either side and the U.S. would continue to play a guarantor role to ensure it remains in force.

"If one side violates the deal, both sides lose," he said.

The accord removes one source of potential conflict between Israel and Iranian-backed Lebanese group Hezbollah and could help alleviate Lebanon's economic crisis.

An offshore energy discovery - while not enough on its own to resolve Lebanon's deep economic problems - would be a major boon, providing badly needed hard currency and possibly one day easing crippling blackouts.

Offshore areas in the eastern Mediterranean and Levant have yielded major gas discoveries in the past decade and interest has grown since Russia's invasion of Ukraine disrupted gas pipeline flows.

In a statement on Thursday, U.S. President Joe Biden said energy in the region "should not be a cause for conflict, but a tool for cooperation, stability, security and prosperity".

Lebanon's powerful armed group Hezbollah has quietly given its green light to the deal. Its head, Sayyed Hassan Nasrallah, was set to give a televised address on Thursday afternoon.