BEIJING/LOS ANGELES (Reuters) -The United States and China on Tuesday will begin charging port fees on ocean shipping firms that move everything from holiday toys to crude oil, making the high seas a key front in the trade war between the world's two largest economies.

Early this year, President Donald Trump's administration announced plans to levy the fees on China-linked ships to loosen that country's grip on the global maritime industry and bolster U.S. shipbuilding. An investigation during former President Joe Biden's administration concluded China uses unfair policies and practices to dominate the global maritime, logistics and shipbuilding sectors, clearing the way for those penalties. 

The U.S. is scheduled to begin collecting those fees on October 14. Analysts expect China-owned container carrier COSCO to be most affected, shouldering nearly half of that segment's expected $3.2 billion cost from those fees in 2026. 

China hit back last week, saying it would impose its own port fees on U.S.-linked vessels, also starting Tuesday. Jefferies analyst Omar Nokta noted that 13% of crude tankers and 11% of container ships in the global fleet would be affected. 

"This tit-for-tat symmetry locks both economies into a spiral of maritime taxation that risks distorting global freight flows," Athens-based Xclusiv Shipbrokers Inc said in a research note. 

In a reprisal against China curbing exports of critical minerals, Trump on Friday threatened to slap additional 100% tariffs on goods from China and put new export controls on "any and all critical software" by November 1. 

Administration officials hours later warned that countries voting in favor of a plan by the United Nations' International Maritime Organization to reduce planet-warming greenhouse gas emissions from ocean shipping this week could face sanctions, port bans, or punitive vessel charges. China has publicly supported the IMO plan. 

"The weaponisation of both trade and environmental policy signals that shipping has moved from being a neutral conduit of global commerce to a direct instrument of statecraft," Xclusiv said.