JOINT BASE ANDREWS: US President Donald Trump said on Tuesday he thinks China can open its markets to American goods, once again asserting he had a good relationship with Chinese leader Xi Jinping.

"I think it's going to happen," Trump said when asked if he thinks Chinese markets could open to US goods. He did not elaborate.

Washington and Beijing have had tense ties for years over issues like trade tariffs, intellectual property, human rights, the origins of the COVID-19 pandemic, cybersecurity and policies towards Taiwan, Hong Kong and Russia's war in Ukraine.

A day earlier Trump threatened to slap a 25 per cent tariff on countries that trade with Iran, risking reopening old wounds with Beijing, Tehran's biggest trading partner.

China condemned Trump's threat and Trump did not provide details on his plan to impose tariffs on countries conducting trade with Iran, which is currently seeing some of its biggest anti-government demonstrations in years.

Trump has been weighing how to respond to the situation in Iran, which fought a 12-day war with US ally Israel last year and whose nuclear sites were bombed by the American military in June.