It’s the latest tit-for-tat tariff spat between the two superpowers following a period of calmed tensions.

China and the US have started charging additional port fees on shipping vessels as trade tensions between the world’s two largest economies soared back, following China’s rare earth export controls in response to new trade restrictions imposed by the Trump administration.

The port fees from both sides went into force on Tuesday and have caused fear among analysts, who say maritime trade has become a key battlefront between the two nations.

The new trade tensions erupted despite a pause in the tariff war, and come as Trump is expected to meet his Chinese counterpart Xi Jinping on the sidelines of the Asia-Pacific Economic Cooperation (APEC) summit in South Korea at the end of this month.

China has accused the US of “double standards” after it threatened to impose a 100 percent tariff on Chinese goods following last week’s rare earth curbs by Beijing.

Here’s what to know about the new port tariffs as the two countries are engaged in ongoing trade negotiations:

What are the port fees imposed by the two sides?

A White House executive order named “Restoring America’s Maritime Dominance” directed the US Trade Representative (USTR) to impose charges on owners and operators of Chinese-built, owned, or operated vessels entering the US by October 14 as follows:

Vess