China on Wednesday expressed "regret" over US President Donald's Trump's decision to withdraw from the Iran nuclear deal and said it would continue its trade and economic cooperation with Iran.

Foreign Ministry spokesman Geng Shuan told reporters in Beijing that "ensuring the integrity and sanctity" of the agreement was important for upholding the international non-proliferation regime and promoting peace and stability in the Middle East.

Trump on Tuesday announced that the U.S. would withdraw from the international deal and restore sanctions on Iran, leaving the future of the agreement in doubt.

China is strongly invested in the agreement and it's unclear what effect Trump's widely-criticised decision to re-impose sanctions will have on its relationship with Tehran.

China was involved in negotiating the agreement as one of the five permanent members of the United Nations Security Council and has long been a close Iranian economic partner, buying about a third of Iran's oil shipments

Geng said China would maintain normal trading ties with Iran.

At the regular briefing with reporters, Geng also said the visit of the North Korean leader Kim Jong Un to China earlier this week was proposed by North Korea.

Xi and Kim held talks on Monday and Tuesday in the northern Chinese port of Dalian, the second time they've met in just over a month following Kim's unannounced trip to Beijing in late March.

That was Kim's first overseas visit since taking power six years ago and a powerful sign of the enduring relationship between the communist neighbours despite years of fraying ties.