US fears Chinese companies are breaking Iran sanctions
The United States has asked the Chinese government to do more to stop Chinese companies helping Iran with its nuclear programme and missile technology.
A senior US official told the BBC that Washington had provided Beijing with a list of firms it believed had been operating in violation of UN sanctions.
Beijing promised it was committed to implementing the sanctions and that it would investigate, the official added.
The US believes Iran is seeking to build a nuclear bomb. Iran denies this.
In June, the UN Security Council imposed a fourth round of sanctions on Iran over its refusal to halt uranium enrichment activities.
The technology used to enrich uranium for use as fuel for nuclear power can also be used to enrich the uranium to the higher level needed to produce a nuclear explosion. Tehran says its intentions are peaceful.
On Sunday, the Washington Post reported that the US intelligence believed several Chinese companies and banks were involved in providing restricted technology to Iran, mostly for its missile programme.
A second official, also speaking anonymously, told the Post that Chinese companies had been discovered selling Iran high-quality carbon fibre, which could help make better centrifuges needed to enrich uranium.
In 2008, Iran allegedly obtained 108 pressure gauges, which are critical to the functioning of a centrifuge, from one Chinese company
On Monday, a senior US official told the BBC that the concerns were raised during a visit to Beijing last month by state department official Robert Einhorn, who oversees the enforcement of sanctions against Iran and North Korea.
The official said the Chinese government had promised it was committed to implementing UN resolutions against Iran, and that Washington expected it to take the appropriate steps to stop any violations.
"We did provide some information to China on specific concerns about individual Chinese companies and the Chinese assured us that they will investigate," state department spokesman PJ Crowley later told reporters.
The BBC's Kim Ghattas in Washington says it is believed that Chinese officials did not authorise the activity of the companies.
When sanctions were passed this summer at the UN, the US and the EU were concerned that Chinese companies would fill the vacuum left by Western companies pulling out of Iran, our correspondent adds.
On Sunday, Iranian President Mahmoud Ahmadinejad said it was ready to resume negotiations with the West on its nuclear programme.