WASHINGTON (AP) — The United States has asked Beijing to do more to stop Chinese companies from providing assistance and expertise to Iran’s efforts to develop nuclear weapons and more powerful missiles, The Washington Post reported Sunday.

According to the newspaper, the Obama administration believes several Chinese companies are violating United Nations sanctions against Iran. An anonymous U.S. official told the paper that Robert Einhorn, the State Department official who oversees the enforcement of sanctions against Iran, gave a “significant list” of such companies and banks to Chinese officials last month.

A Chinese Embassy spokesman said his country would investigate the U.S. charges.

The U.S. official did not name any of the Chinese companies or say how many were on the list. But he told the Post that U.S. intelligence believes several Chinese companies and banks were involved in providing restricted technology to Iran, mostly for its missile program.

A second official, also speaking anonymously, told the Post that Chinese companies had been discovered selling Iran material that could be used to make better centrifuges. Those are used to enrich uranium that could be used in a nuclear device.

In June, the U.N. imposed a fourth round of sanctions on Iran over its refusal to halt uranium enrichment.

Iranian President Mahmoud Ahmadinejad on Sunday endorsed the idea of new talks with the international community over his country’s nuclear program, but warned that negotiations would fail if the West does not clearly come out against Israel’s suspected nuclear arsenal.

“They have come and said, ‘We will negotiate,'” Ahmadinejad said. “We say, ‘All right, we will negotiate with you.'”

In Washington, State Department spokesman P.J. Crowley said it was up to Iran to set a date.

“Iran says it is ready to talk,” Crowley said. “Now it needs to commit to a date. Iran knows the phone number. We are awaiting Iran’s formal response.”