Lebanon has formally approved a landmark U.S.-brokered agreement laying out the country’s maritime boundary with Israel, the country's top negotiator told reporters on Thursday.

Lebanese President Michel Aoun on Thursday signed a letter approving the historic agreement with Israel.

Israel was set to follow suit in approving the deal, which marks a diplomatic departure from decades of hostility, later in the day.

The agreement opens the way for offshore energy exploration and removes one source of potential conflict between Israel and the Iranian-backed Lebanese group Hezbollah and could help alleviate Lebanon's economic crisis.

Speaking from the presidential palace, negotiator Elias Bou Saab said the agreement marked the beginning of "a new era" and that the letter would be submitted to U.S. officials at Lebanon's southernmost border point of Naqoura later on Thursday.

Hailed by all three parties as a historic achievement, the deal will be signed separately in Jerusalem by Israeli Prime Minister Yair Lapid following his cabinet's approval.

Asked what happens in case of a violation by either of the sides, which remain technically at war, U.S. envoy Amos Hochstein said the U.S. would remain a guarantor to help resolve any disputes.

"If one side violates the deal, both sides lose," Hochstein told reporters.

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.