Such transactions are believed to have helped keep Iran's economy afloat, despite US sanctions to put pressure on Tehran to curb its nuclear programme.
“We are aware of the purchases that Chinese companies are making of Iranian oil,” a senior US official told Reuters.
“We have used our sanctions authorities to respond to Iranian sanctions evasion, including those doing business with China, and will continue to do so if necessary.”
The approach has been diplomatic, believed to be a more effective path forward, the official said.
The Iran shipments was one of the issues raised by US Deputy Secretary of State Wendy Sherman when she visited China in July, a European official told Reuters.
China has been protecting Iran, said the official, and suggested one of the main issues for the West is the size of the purchases.
The US State Department did not immediately respond to requests for comment.
Commodity analytics firm Kpler estimates that year-to-date Chinese oil imports from Iran averaged 553,000 barrels a day in August.
Indirect US-Iranian talks about reviving the 2015 nuclear deal that began in April adjourned in June, two days after hardliner Ebrahim Raisi was elected president of Iran.
He replaced Hassan Rouhani, whose administration had negotiated the agreement.
Under the deal, Iran agreed to place limits on its uranium enrichment programme, which is one possible pathway to develop the fissile material for a nuclear weapon, in exchange for the easing of US, UN and EU economic sanctions.
Iran has denied seeking a nuclear bomb.
In 2018, Donald Trump, US president at the time, abandoned the agreement and re-imposed harsh economic sanctions that hurt Iran's economy.
After waiting for about a year, Iran responded to Mr Trump's move by starting to carry out some of the nuclear activities that it had agreed to curb under the Joint Comprehensive Plan of Action.
Trump's successor President Joe Biden has said he was putting “diplomacy first” with Iran but if negotiations fail, he would be prepared to turn to other unspecified options.
Iran's Foreign Minister Hossein Amirabdollahian said last week Tehran will return to talks on resuming compliance with the nuclear deal “very soon”, but gave no specific date.
It is unclear how receptive China may be to any US diplomatic overture on Iran.
US-Sino relations have sunk to their worst state in decades this year, with scant progress on issues from human rights to transparency over the origins of Covid-19.
Last week, the Chinese foreign ministry put the onus on the US, rather than Iran.
“As the one that started the new round of tensions in the Iranian nuclear situation, the US should redress its wrong policy of maximum pressure on Iran, lift all illegal sanctions on Iran and measures of long-arm jurisdiction on third parties, and work to resume negotiations and achieve outcomes at an early date,” said the ministry.