China's Huawei has long denied violating American trade sanctions on Iran.
But newly obtained documents seen by Reuters, suggest otherwise.
Huawei produced internal company records in 2010, that show it was involved in sending prohibited U.S. computer equipment to Iran's largest mobile-phone operator.
The documents reveal two Huawei packing lists, dated December 2010.
They included computer equipment made by Hewlett-Packard.
Another document, dated two months later, said: "Currently the equipment is delivered to Tehran, and waiting for the custom clearance."
It's the strongest documentary evidence to date of Huawei's involvement in alleged trade sanction violations, and could bolster Washington's campaign against the firm.
The U.S. is trying to persuade allies to avoid using the world's leading telecommunications-equipment maker in their 5G projects.
Separately, U.S. authorities are battling Huawei on a legal front.
The newly obtained documents involve a multi-million dollar telecommunications project in Iran that figures prominently in an ongoing criminal case.
Washington brought the case against Huawei and its chief financial officer, Meng Wanzhou.
The daughter of Huawei's founder, Meng has been fighting extradition from Canada to the U.S. since her arrest in 2018.
Both Huawei and Meng have denied the charges, which involve bank fraud, wire fraud and other allegations.