Top US officials expressed concern Tuesday at reports that Chinese telecoms giant Huawei is set to build a new broadband network in South Korea that Washington sees as a possible spying risk.

Huawei, barred from projects in the United States and Australia over national security concerns, was reportedly selected as a subcontractor for LGU+, a subsidiary of South Korea's LG Corp, to build a Long Term Evolution (LTE) network.

But US defense and intelligence officials and key lawmakers on Capitol Hill are expressing concerns over the deal, amid fears it could allow Chinese spies access to sensitive communications and even pose an eavesdropping threat to US forces in South Korea and elsewhere.

A senior US official said that Washington was "concerned" about the potential project, because Huawei was excluded from a project to provide wireless broadband services in the United States over similar national security concerns.

A defense official meanwhile said that there were also concerns at the Pentagon and that US force commanders in South Korea had been asked to provide an "assessment" of the potential impact of the deal.

Officials would not say if they had formally raised those concerns directly with the South Korean government.

News of US red flags surfaced as Vice President Joe Biden travels on an East Asia tour, including stops in both China and South Korea, which is being overshadowed by tensions over Beijing's self declared air defense zone in the East China Sea.

It also comes amid prickly relations between the two key US allies South Korea and Japan -- and with lawmakers in Washington acutely concerned at Chinese surveillance of US activities on the American mainland and abroad.

But signs of US concern may anger China -- and leave Washington on sensitive political ground given leaks by fugitive intelligence contractor Edward Snowden about a massive American worldwide eavesdropping operation by the National Security Agency.

Two key senators charged with national security issues, raised the South Korea deal in a letter to Defense Secretary Chuck Hagel, Director of National Intelligence James Clapper and Secretary of State John Kerry last week.

"Maintaining the integrity of telecommunications infrastructure is critical to the operational effectiveness of this important security alliance (between the United States and South Korea)," wrote Democratic senators Dianne Feinstein and Robert Menendez in the letter, obatained by AFP.

"In this context the recent press reports that Huawei has been selected to develop and/or supply the Republic of Korea's advanced LTE telecommunications backbone raise serious questions and potential security concerns."

The senators, chairs of the Senate Foreign Relations Committee and Select Committee on Intelligence respectively, asked for an administration assessment of the potential security threats from the deal and whether the US government had discussed the importance of "network integrity" with South Korea.

The United States, Britain, and Australia have all raised concerns that Huawei's alleged ties to the Chinese state could see telecoms equipment supplied by the company used for spying and cyber-attacks.

Huawei denies it has any direct links to the Chinese state, but Congress last year called for its exclusion from US government contracts, while Australia has barred it from involvement in the country's new broadband network.