Membership to the World Trade Organization has benefited the U.S. and China more than any other nation, a study published Monday found, as the two powers seek to defuse a monthslong trade conflict.

The Bertelsmann Foundation research showed WTO membership has boosted the U.S.' gross domestic product by $87 billion in the 25 years since the country joined.

China, which only became a member in 2001, has gained $86 billion, while Germany added $66 billion.

"Even if no organization is perfect, anyone who believes they can rely on a system of bilateral trade agreements instead of the WTO risks enormous losses of prosperity in international trade," Bertelsmann trade expert Christian Bluth said.

With 164 member countries, the WTO will celebrate a rocky 25th anniversary on Jan. 1, 2020.

Washington refuses to name new judges to its appellate body, blocking arbitration of trade disputes.

Washington and Beijing have nonetheless struck a truce in their tit-for-tat tariff war, hoping to sign a preliminary trade deal in January.

Around the world, Bertelsmann found WTO members gained on average 4.5% of GDP from membership. The total increase reached $855 billion or 1% of global output, the study showed.

And around the world, WTO members' exports increased an average of 14% between 1980 and 2016, while non-members' exports fell almost 6%.

So far, "nations with strong exports and production are the main beneficiaries," the Bertelsmann Foundation said in a statement, pointing to countries such as South Korea and Mexico as further winners.

On the other hand, European countries with smaller manufacturing sectors have not been able to make such large gains from WTO membership.

France's output was boosted by $25 billion, while Britain's added $22 billion — both well below the average increase of 4.5% of GDP.

European Commission President Ursula von der Leyen has said she planned to meet U.S. President Donald Trump "at the beginning of 2020," with trade one of the major issues on the table.