China has called the US naval blockade of Iranian ports irresponsible and dangerous.
Beijing's foreign ministry said the move would undermine the already fragile ceasefire agreement and further jeopardise the safety of ships passing through the Strait of Hormuz, a critical waterway Iran effectively closed in response to US-Israeli strikes.
The blockade came into force on Monday, a day after peace talks between the US and Iran broke down in Pakistan.
US President Donald Trump has said the measure is intended to force Iran to give up its nuclear ambitions. Analysts say the move is also designed to pressure China - the biggest buyer of Iranian oil - to encourage Tehran to open the strait.
Iran's ambassador to the UN has described the US blockade as a grave violation of Tehran's sovereignty.
Up until now, Chinese vessels have been among the few managing to make their way through the strait. It is not clear whether they had to pay a toll to Iran in order to do so.
The US blockade could potentially cut off China's supply and have far-reaching consequences for its economy.
China urges all parties to abide by the ceasefire arrangements, focus on the general direction of dialogue and peace talks, take practical actions to promote the easing of the regional situation, and restore normal traffic in the strait as soon as possible.
Trump has threatened to impose a 50% tariff on China's goods if military assistance was provided to Tehran.
If the US insists on using this as an excuse to impose additional tariffs on China, China will definitely take resolute countermeasures, Guo said.
A delicate and fragile ceasefire between the US and Iran remains in place, having been implemented on 8 April. Major points of contention between the two sides include the status of the Strait of Hormuz, and whether Lebanon is included in the agreement.


















