Chinese Foreign Minister Wang Yi hosted Iran's top diplomat Mohammad Javad Zarif for talks in what Wang described as their seventh round this year.
China took part as a member of the P5+1 -- the five permanent members of the United Nations Security Council and Germany -- in the talks with Iran that yielded the nuclear agreement in July.
In exchange for lifting painful economic sanctions, the hard-fought deal limits Tehran's nuclear programme, which was seen by the United States, Israel and others as ultimately intended to develop atomic weapons.
In an attempt to choke off Iran's oil exports the United States from 2012 threatened to penalise foreign financial institutions over transactions with Iran's central bank, which handles sales of the country's key export.
China significantly cut back on purchases of Iranian crude, earning an exemption from the sanctions as did several other countries.
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Wang said energy was a "traditional area" of cooperation between Beijing and Tehran, and their overall relationship "will have new prospects after the comprehensive Iran nuclear deal is implemented".
At that point, Zarif said, "illegal sanctions imposed upon Iran by Western countries will be lifted, a lot of Chinese companies will have more chances to cooperate with Iran".
"China and Iran are reliable partners in energy, trade and economy," he added.
On broader issues, both sides stressed the need to work together to solve hotspot security problems through political means.
"We would like to cooperate with China on issues in Yemen, Syria and the Middle East, seeking a political solution," Zarif said.
Iran is a major backer of President Bashar al-Assad of Syria, which is battling Islamic State and other groups trying to overthrow his regime.
China and Russia have cooperated at the Security Council to block some attempts by the US and other Western countries to punish Syria.