The US State Department said Thursday it was mulling a request from US lawmakers to probe whether China's Huawei Technologies and other telecom firms are violating US sanctions on Iran.

Last month, six lawmakers sent a letter to Secretary of State Hillary Clinton, urging her to look into press reports that Huawei had sold technology to Iran allowing Tehran to monitor people's locations via their cell phones.

"Such services may be a critical tool for the Iranian government in disrupting and restricting the speech of its people," said the lawmakers, including senior Republican Senator Jon Kyl.

Spokeswoman Victoria Nuland told reporters on Thursday that the State Department had received the letter, dated December 22, and was "following up on it."

"This is a complex issue. It's going to take time," Nuland said.

"As we have already explained to the Chinese government, if we assess that the company involved was engaged in sanctionable activity involving the export of technology to Iran... we will have to take appropriate action."

Huawei, founded by a former People's Liberation Army engineer, produces technology that is used to build mobile phone networks around the world. It has repeatedly denied any links to the Chinese military.

The lawmakers hailed Huawei's decision, announced last month, to scale back its activities in Iran, due to what the Shenzhen-based company called the "increasingly complex situation" in the Islamic republic.

But it warned that the company's "previous actions and continuing service of existing service with Iranian clients may violate" US sanctions.

US sanctions bar any company exporting sensitive telecommunications technology to Iran from entering into a contract with the US government.

The lawmakers cited a report in the Wall Street Journal which said other firms including Telefon AB L.M. Ericsson and Nokia Siemens Networks also provided technology to Iranian carriers that could be used for monitoring.

Another report from Bloomberg, cited by the lawmakers, also named Creativity Software Ltd. and AdaptiveMobile Security Ltd. as marketing or providing similar technology to Iranian state-run telecom operators.

The lawmakers said that if the State Department were to identify companies in violation of US sanctions, it would "send a powerful message to the commercial accomplices of the Iranian regime that their actions will not go overlooked."