JERUSALEM — Prime Minister Benjamin Netanyahu of Israel on Thursday announced final government approval for a long-delayed deal that will let an American-Israeli energy partnership develop an offshore natural gas field.

The deal adds to the industry momentum in the eastern Mediterranean Sea, which could emerge as a big new source of natural gas, which is increasingly in demand around the world as a cleaner-energy alternative to coal.

Although the big gas field in question is expected to help make Israel an energy exporter for the first time, Mr. Netanyahu on Thursday emphasized the national security implications, saying development of the field would give the country more energy options. Authorizing the deal, Mr. Netanyahu dismissed objections by political opponents and from the Israeli antitrust commissioner, who had opposed the agreement on the grounds that it would give too much market power to the team led by the American company, Noble Energy.

The American company, which has received political support from the Obama administration, and its partners from the Delek Group, an Israeli conglomerate, have made major gas discoveries in Israeli territory in the Mediterranean in recent years. They already produce gas from a field called Tamar and now intend to develop the much bigger field, which is called Leviathan, and to expand Tamar’s production.