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 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.
