Iran and China have signed a 25-year cooperation agreement on Saturday as the US rivals move closer together.

China is Iran’s leading trade partner and was the biggest buyer of Iranian oil before then US president Donald Trump reimposed sanctions in 2018, abandoning a nuclear agreement with Tehran.

The pact was signed by Chinese Foreign Minister Wang Yi, who arrived in Tehran late Friday.

"We believe this document can be very effective in deepening" Iran-China relations, the spokesman said.

Xi and his Iranian counterpart Hassan Rouhani agreed then to establish a roadmap for "reciprocal investments in the fields of transport, ports, energy, industry and services."

A signing ceremony was hosted by Iranian Foreign Minister Mohammad Javad Zarif.

Last year in July, controversy erupted over the proposed deal after former president Mahmoud Ahmadinejad charged that negotiations were being held for a "new, 25-year agreement with a foreign country" without the knowledge of the Iranian people.

The foreign minister was heckled in parliament when he assured lawmakers there was "nothing secret" in the proposed deal, which he promised would be publicly announced "once it has been finalised".

The government has yet to honour that promise and few details have been made public.