In a meeting on Friday in the city of Wuxi, in Jiangsu province, Chinese Foreign Minister Wang Yi also backed efforts to revive a 2015 nuclear deal between major powers and Iran.
A summary of the meeting between Wang and Iranian Foreign Minister Hossein Amirabdollahian was posted on China's foreign ministry website on Saturday.
Wang, who is also State Councillor, said the US bore primary responsibility for the ongoing difficulties with Iran, having unilaterally withdrawn from a 2015 nuclear deal between the major powers and Iran.
Under the terms of that deal, in return for the lifting of international sanctions, Iran would limit uranium enrichment activity, making it harder to develop nuclear arms - although Tehran denies having plans for nuclear weapons.
Wang said China would firmly support a resumption on negotiations on a nuclear pact.
But he said China firmly opposes unilateral sanctions against Iran, political manipulation through topics including human rights, and interference in the internal affairs of Iran and other regional countries.
The United States reimposed sanctions that badly damaged Iran's economy after withdrawing from the nuclear pact in 2018, saying the terms did not do enough to curb Iran's nuclear activities, ballistic missile program and regional influence.
A year later, Iran began to gradually breach the accord, rebuilding stockpiles of enriched uranium, refining it to higher fissile purity and installing advanced centrifuges to speed up output.
China and Iran, both subject to US sanctions, signed the 25-year cooperation agreement last March, bringing Iran into China' Belt and Road Initiative, a multi-trillion-dollar infrastructure scheme intended to stretch from East Asia to Europe.
The project aims to significantly expand China's economic and political influence, and has raised concerns in the United States and elsewhere.
The foreign ministry summary said the agreement would deepen Sino-Iranian cooperation in areas including energy, infrastructure, agriculture, health care and culture, as well as cyber security and cooperation with other countries.
Iran and the US remain locked in talks over whether a compromise can be found to renew the deal and dispel fears of a wider Middle East War. A source close to negotiations said on Friday that many issues remain unresolved.
Wang, who earlier in the week met with several counterparts from Gulf Arab countries concerned about the potential threat from Iran, also said China hopes to set up a dialogue mechanism with Gulf countries to discuss regional security issues.