BEIJING, Jan 13 - U.S. President Donald Trump's threat to slap a 25% tariff on countries that trade with Iran risks reopening old wounds with Beijing, Tehran's biggest trading partner.

Iran became a flashpoint in U.S.-China ties during Trump's 2017-21 first term as president as Washington tightened sanctions on Tehran and put China's Huawei, accused of selling technology to the Islamic Republic, in its crosshairs.