London-listed firm Energean on Sunday began testing pipes between Israel and the Karish offshore gas field, a key step towards production from the eastern Mediterranean site, a source of friction between neighbours Israel and Lebanon.
Israel has maintained that Karish falls entirely within its territory and is not a subject of negotiation at ongoing, US-mediated maritime border talks with Lebanon.
The two countries remain technically at war.
Beirut has reportedly made claims to parts of Karish, and the Iran-backed Shiite group Hezbollah, which holds huge influence in Lebanon, has previously threatened attacks if Israel began production from the field.
In a statement Sunday, Energean said that "following approval received from the Israeli Ministry of Energy to start certain testing procedures, the flow of gas from onshore to the FPSO has commenced", referring to the Karish floating production storage offloading facility.
The tests, set to take a number of weeks, were "an important step" towards extracting gas from the Karish, Energean said.
Lebanon and Israel have engaged in on-off indirect talks since 2020 to delineate their Mediterranean border, which could allow both countries to boost offshore natural gas exploration.
A draft agreement floated by US envoy Amos Hochstein aims to settle competing claims over offshore gas fields and was delivered to Lebanese and Israeli officials in recent days.
Israel had welcomed the terms set out by Hochstein and said they would be subjected to legal review, but gave no indication if it sought substantive changes.
Lebanon presented its response to Washington's proposal on Tuesday.
Israel said two days later that it planned to reject a proposed Lebanese amendment, even if that jeopardises a possible agreement.
Israel reiterated this week that production at Karish would begin as soon as possible, regardless of Lebanon's demands.
Two Lebanese officials involved in the talks told AFP on Sunday the US mediator had informed Beirut that the operation at Karish was only a test.
Negotiations on the maritime border are still going on, one official said.
On Saturday, the French foreign ministry said Paris was "actively contributing to the American mediation".
Under the terms of the US draft agreement leaked to the press, all of Karish would fall under Israeli control, while Qana, another potential gas field, would be divided but its exploitation would be under Lebanon's control.
French company Total would be licensed to search for gas in the Qana field, and Israel would receive a share of future revenue.