GENEVA - The United States has become the first major fishing nation to ratify a deal to cut subsidies contributing to overfishing, the World Trade Organization chief said on Wednesday.

The trade watchdog's 164 members reached the deal, which aims to cut billions of dollars in harmful subsidies that are emptying the ocean of marine life, last year.

But it needs two-thirds of the members to sign on to take effect and only a handful of smaller countries have accepted it so far. The United States ratified it on Tuesday, WTO Director-General Ngozi Okonjo-Iweala wrote on Twitter.

"Hope others will follow suit. Let's get it done by MC13!," Okonjo-Iweala said, referring to the next major conference of the trade watchdog in 2024, where outstanding parts of the deal will be discussed.

"We are proud to be among the first WTO members to accept this agreement, which is the first ever multilateral trade agreement with environmental sustainability at its core," U.S. Trade Representative Katherine Tai said in an emailed statement.

Global subsidies are estimated at $35.4 billion, according to a 2019 study published in Marine Policy. The top five subsidisers are China, the EU, the United States, South Korea and Japan, it said, although not all of them are considered "harmful" and within the scope of the WTO deal.

(Reporting by Emma Farge; Editing by Gerry Doyle)