President Donald Trump’s threat of 25% tariffs on countries doing business with Iran has raised the risk of derailing Washington’s fragile trade deal with Beijing — Tehran’s largest trading partner.

Trump said Monday night stateside that the U.S. will start charging a 25% tariff on imports from countries that do business with Iran. The order is “effective immediately,” he said in a Truth Social post.

The world’s top two economies had secured an interim trade deal in late October that saw a roll back of punitive U.S. tariffs on China, while Beijing paused its sweeping rare earth export controls.

In response to Trump’s tariff threat China said it “firmly opposes any illicit unilateral sanctions and long-arm jurisdiction,” while warning that it would take “all necessary measures” to defend its interests, according to a post on X by a spokesperson for the Chinese Embassy in the U.S.

If Trump is serious about the 25% rate, “that is a massive escalation from current tariff levels,” said Deborah Elms, head of trade policy at the Hinrich Foundation.