Lebanon and Israel have been locked in U.S.-mediated negotiations to delineate a shared maritime border that would help determine which oil and gas resources belong to which country and pave the way for more exploration.

Hochstein has been shuttling between Lebanon and Israel - enemy states with a history of conflict - in a bid to forge a compromise.

The head of Lebanon's Iran-backed Hezbollah, which has warned against any Israeli exploration and extraction in the disputed waters, said the receipt of the letter was "a very important step".

Sayyed Hassan Nasrallah said in a televised speech that the coming days would be crucial.

Lebanese officials have demanded a written version of the proposal before providing a final answer, a political source with knowledge of the talks said.

A deal could defuse a potential source of conflict between Israel and the heavily armed Hezbollah.

(Reporting by Enas Alashray and Laila Bassam and Moataz Mohamed;Editing by Gareth Jones and Nick Macfie)