China will prioritise its energy and financial cooperation with Iran, as the recent nuclear deal means there is now even more opportunity to work together, Chinese President Xi Jinping told his Iranian counterpart Hassan Rouhani.

China and Iran have close diplomatic, economic, trade and energy ties, and China has been active in pushing both the United States and Iran to reach agreement on the nuclear issue.

Under the multilateral deal, agreed in July, sanctions imposed by the US, European Union and United Nations will be lifted in return for Iran agreeing to long-term curbs on a nuclear programme that the West has suspected was aimed at creating a nuclear bomb.

Meeting in New York on the sidelines of the UN, Xi told Rouhani that once the nuclear agreement was put in force "Iran will have ever more opportunity for foreign cooperation, and Sino-Iran ties will face a new development opportunity", China's Foreign Ministry said on Tuesday.

China wants increased cooperation in the fields of railways, roads, iron and steel, auto manufacturing, electricity and high-technology, Xi said.

"(We) must prioritise energy and financial cooperation," he added, without elaborating.

China is the biggest customer of Iranian oil.

Last week, a senior Chinese envoy offered Iran help with upgrading its manufacturing technology to boost its economy.

China had long railed against unilateral sanctions imposed on Iran by the United States and Europe, though it has supported UN ones, and had denounced threats of force. - Reuters