Iranian President Hassan Rouhani has warned the United States against any attempt to stop Tehran's oil trade, threatening to block the strategically important Strait of Hormuz. Their arrival means state carrier Iran Air has received 13 of the 20 it ordered from the French-Italian manufacturer in April 2017.
In May, the United States pulled out of a 2015 deal between world powers and Tehran under which worldwide sanctions on Iran were lifted in return for curbs on its nuclear programme.
The Chinese position comes as the debate over the Iranian deal butts up against tense U.S.
In a statement, Captain Bill Urban, the military's Central Command spokesman, said the United States was "aware of an increase" in Iranian naval operations in the Gulf, the Strait of Hormuz and the Gulf of Oman.
Iran is also said to have started to offer India cargo insurance and tankers operated by Iranian companies as some Indian insurers have refused to cover oil cargoes from Iran in the face of the returning US sanctions on Tehran.
Iran had hoped the lifting of sanctions would allow it to replace its aging commercial airline fleet, but the US withdrawal has halted billion-dollar deals struck with Airbus and Boeing.More news: US President Donald Trump hits back at LeBron James over CNN interview
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Iran's economy has rapidly deteriorated in recent months, fueling protests. The Iranian rial has fallen to 99,000 to the US dollar despite a government-imposed rate of 44,000. So far, social media reports suggest the current protests are far from the scale of the unrest seen in December and January, when at least 25 people were killed in demonstrations that spread to dozens of towns and cities.
These will take effect on August 6, November 4 and November 5.
On Friday, China said it will not cut Iranian oil imports despite President Trump threatening to sanction any countries that fail to stop importing Iranian oil by November 4, a symbolic date coinciding with the start of what became known as the Iranian hostage crisis.
Tamar Essner, an analyst at Nasdaq Inc., said there are "no signs whatsoever that this trade war is going to clear up anytime soon", and that has "caused investors to continue to trim net length, take profits, and de-risk that position with the sense that oil's upside is limited unless there's material reduction in Iranian barrels".
On Sunday, the semi-official Tasnim news agency quoted Guard spokesman Gen. Ramezan Sharif confirming that its forces held a military drill in the Persian Gulf and the strait.
The exercise could shut down of the Strait of Hormuz, a crucial Gulf passage for shipments of numerous commodities. He said the exercise, which he described as an annual drill, was aimed at maintaining the security of the worldwide waterway.