A staff member of the House of Representatives Intelligence Committee said the panel has been receiving "dozens and dozens" of calls from current and former employees and customers reporting supposedly suspicious equipment behavior, chiefly involving Huawei.

"I don't think the companies should expect our attention to stop," the staff member said, adding that the panel would follow up on new leads.

In a report issued on Monday after an 11-month investigation, the House committee warned US industry that Beijing could use equipment made by the two companies to spy on certain communications and threaten vital systems through computerized links. It urged network providers to seek other vendors.

The report also advised officials to to block any future business tie-ups involving Huawei or ZTE and US companies.

Huawei, the world's second-largest maker of routers and other telecom gear, and ZTE, the fifth-largest, both rejected the allegations. China's Commerce Ministry said the US committee had "made groundless accusations against China."

Adding to Huawei's problems, Canada indicated on Tuesday that it would exclude Huawei from firms allowed to build a secure Canadian government communications network, citing possible security risks.

In March, Australia barred Huawei from seeking contracts for the country's National Broadband Network due to cyber security concerns.

By contrast, the European Commission has delayed a trade case against the two Chinese telecom equipment makers, easing tensions between the European Union and its second-biggest trading partner.

The US panel's 52-page report did not present concrete evidence that the companies' equipment had been used for espionage, but a classified annex provides "significantly more information adding to the committee's concerns," it said.

Current and former US intelligence officials said Huawei and ZTE, both based in Shenzhen in southern China, pose potential national security threats, but there did not appear to be a consensus about whether security breaches involving their equipment had been confirmed.

One former US official said there were "smoking guns" that justified suspicions about Huawei, noting that the defense industry was a primary target. Another former senior US intelligence official said the threat of illegal eavesdropping may be more theoretical than actual.

On Monday, House Intelligence Committee Chairman Mike Rogers referred to alleged instances of "beaconing" of information to China, though he did not name any specific users of Huawei's equipment that had been affected.

In Britain, the Chinese firm's gear sits at the core of BT's national phone network and is being used to build 4G mobile broadband networks.