Chinese companies have been breaking the UN embargoes against Iran by aiding in Tehran's nuclear programme, the US government has concluded.

President Barack Obama's administration believes Chinese firms have assisted in missile technology and in the development of nuclear weapons, the Washington Post reported Monday, citing unnamed US officials.

The US has asked Beijing to halt the cooperation with Tehran, and last month a State Department delegation to China provided the government with a list of companies and banks involved in the efforts.

The US believes that the companies violated UN sanctions, but that the government had not authorised the activities, the report said.

A Chinese embassy spokesman said Beijing was investigating the allegations.

The UN Security Council approved a fourth round of sanctions against Iran in June, and the US, European Union and others also put in place further new sanctions.

Western nations fear Tehran's nuclear programme is aimed at acquiring weapons, but Iran insists it is for peaceful purposes.

In recent days, Iran has signalled its willingness to return to talks, and President Mahmoud Ahmadinejad Sunday welcomed the resumption of nuclear talks with world powers but reiterated that Iran would make no compromise whatsoever on its nuclear programmes.

"Iran welcomes talks, but the other side should not think that we will resume the talks from the position of weakness due to the sanctions (imposed on Iran)," Ahmadinejad said in a speech in the city of Ardebil in northwestern Iran.