Diplomatic observers said the US had paved the way for strategic cooperation between China and Iran, given that both remain under threat from Washington.

“Iran would like China to address the Iranian security and economic issues at [this week’s] Group of 20 summit in Osaka,” said Mohsen Shariatinia, an assistant professor of international relations at the Shahid Beheshti National University in Tehran. “China’s proactive role in supporting JCPOA is in Iran’s and China’s interest, to preserve stability in the region.

“The tension Iran is facing right now is the highest level in the last decade … Iran wants Beijing to politically and strategically support Tehran.”

Trump ‘will be Iran’s best friend’ if it renounces nuclear arms

Beijing’s influence in Iran is largely political rather than economic, given that trade with Iran is affected by the US sanctions regime.

But China is Iran’s biggest oil customer, with imports in April reaching their highest level since August, as buyers rushed shipments through before the US ended sanctions waivers in early May, according to data from the market analysts Refinitiv.

The US reimposed sanctions on Iran in November but gave import waivers to eight countries, including China. In May, aiming to cut Iran’s sales to zero, Washington ended the sanctions waivers, saying no governments would be exempt from its curbs on Iranian oil imports.

China’s exports to Iran fell nearly 70 per cent – from about US$1.2 billion to US$391 million – in the two months after the US also reimposed secondary sanctions on countries defying its Iran sanctions, according to Chinese customs data.

Beijing is trying to forge a united diplomatic front with other countries that oppose the American withdrawal from the JCPOA.

Senior officials from China are set to meet those from Iran, France, Germany, Britain and Russia on Thursday to discuss ways to save the nuclear deal.

“China is the only country that clearly defies sanctions against Iran, and explicitly said it would continue its political and economic ties with Iran,” Shariatinia said. “It also supported Iran in the UN Security Council. Iran appreciates China’s support.”

But China’s political influence over Iranian issues also brings dangers as it, too, faces pressure from the US on trade, according to Chinese experts.

“On the JCPOA, although China has a strong interest in saving the deal and defusing another major nuclear crisis, it does not feel it has decisive leverage over either Iran or the United States,” Zhao Tong, a fellow at the Carnegie-Tsinghua Centre for Global Policy, said.

“As a result, China may raise the Iran nuclear issue in Osaka, but I would not expect it to devote major diplomatic resources to battling the American position.”

Beijing must walk a tightrope in managing its relations with Iran and Saudi Arabia – a rival of Tehran but an ally of the US. China is Saudi Arabia’s largest trading partner, with goods totalling US$63 billion last year. In 2017, during the last major state visit to Beijing by Saudi Arabia’s King Salman, the two sides signed deals worth about US$65 billion, mostly related to energy and technology.

Observers have said Beijing is unlikely to stand up for Iran in any prominent way, but may offer diplomatic and security support.

Stronger military ties might also be possible, they said. Since 2014, 96 per cent of Iran’s weapons imports have come from Russia, while the rest were provided by China.

“Because of Iran’s strategic importance in the Middle East, China is likely to develop stronger military ties with it in the future, but this probably would play out gradually over time rather than in any dramatic change overnight,” Zhao said.

Adam Ni, a China researcher at Macquarie University in Sydney, also said military ties between Iran and China would strengthen as their relations became more mutually beneficial.

“China sees Iran as an important regional partner as well as a country that is also under pressure from the US,” he said.

“There is an impetus for Beijing to tighten its relations with countries such as Russia, Iran and North Korea, given the current strategic context. For Beijing, building good relations with these countries provides leverage in its own dealings with the US. Tacit diplomatic and economic support is likely.”

Former US defence secretary James Mattis last year called China and Russia “revisionist powers” and said the focus of US military strategy was shifting away from terrorism and back towards the “great power competition”.