The United States unveiled new port fees on Chinese built and operated ships on Thursday.

The move -- which stems from a probe launched under the prior administration -- comes as the United States and China are locked in a major trade war over US President Donald Trump's tariffs.

"Ships and shipping are vital to American economic security and the free flow of commerce," US Trade Representative Jamieson Greer said in a statement announcing the new fees, most of which will begin in mid-October.

Under the new rule, per tonnage fees will apply to each Chinese-linked ship's US voyage up to five times per year.

There will be separate fees for Chinese operated ships and Chinese built ships, and both will gradually increase over subsequent years.

All non-US built car carrier vessels will also be hit with a fee beginning in 180 days.

It also introduces new fees for liquified natural gas carriers, though those do not take effect for three years.

A fact sheet accompanying the announcement said fees will not cover "Great Lakes or Caribbean shipping, shipping to and from US territories, or bulk commodity exports on ships that arrive in the United States empty." (AFP)