Prime Minister David Cameron was forced to defend the UK's decision to allow China's Huawei to build huge parts of the country's telecoms infrastructure, as US senators expressed concern about the company's role internationally.

Asked whether he was concerned that the UK's cyber-security might be compromised by the involvement of Chinese firms such as Huawei in Britain's telecoms infrastructure, Mr Cameron told Channel 4 News: "We have a proper system in the UK for examining whether investments in the UK are pro-competitive and whether they're in the national interest. We also have a very good way of defending ourselves in terms of cyber-security, I think we're one of the most advanced countries in the world in terms of the action we're taking on cyber-security, and I've made sure we've put extra money into it."

He was questioned after The Times reported that the UK's national security adviser will approve Huawei's systems after vetting them on national security grounds.

The checks were carried out after MPs expressed concerns in October 2012 about potential links between Huawei and the Chinese military. Some experts raised fears that Huawei's systems could have secret "back doors" in the software or hardware which could be exploited by foreign spies to tap calls or disable systems.

In the US, the heads of two US Senate committees have expressed concerns to the Obama administration about a recent network supply deal, under which Huawei Technologies will build telecoms systems in South Korea - an ally of the US.

Critical operations

The role of the Chinese company, which has taken a dominant role in building mobile and fixed telecoms infrastructure, has become contentious in recent years as its influence has grown. Australia excluded Huawei from its National Broadband Network project, and Canada has considered a similar move.

Democratic Senators Robert Menendez, chairman of the Senate Foreign Relations Committee, and Dianne Feinstein, chairwoman of the Senate Intelligence Committee, said media reports on Huawei's supply deal with LG Uplus raised concerns in light of the close security alliance between the United States and South Korea.

"Maintaining the integrity of telecommunications infrastructure is critical to the operational effectiveness of this important security alliance," they said in a letter to Secretary of Defense Chuck Hagel, Secretary of State John Kerry and James Clapper, President Barack Obama's Director of National Intelligence.

South Korea still hosts some 28,000 US soldiers to deter potential provocation from North Korea; the two are officially still at war. US Vice President Joe Biden is due to visit the South Korean capital Seoul later this week as part of a broader Asia trip.

But Huawei reacted angrily to the senators' letter. "Our gear is world-proven and trusted, connecting almost one-third of the world's population. The motivations of those that might groundlessly purport otherwise are puzzling," the company said in a statement. "Huawei has a proven track record of providing secure products and solutions to our customers. There has never been one incident where Huawei's commitment to security has ever been called into question."

LG Uplus, the country's third-largest mobile carrier, added Huawei to its fourth-generation mobile network vendor list in October to boost competition. It was already working with Samsung Electronics and with Ericsson and Nokia's telecoms gear unit.

Security concerns

"There is security concern when you purchase telecoms equipment from foreign suppliers. It's not just limited to one specific company," said Lee Dong-ho, an official in charge of telecoms network regulation at the Ministry of Science, ICT and Future Planning. "But they are providing equipment in accordance with local regulations, and we also have authorities and proper systems in place aimed at monitoring any security breaches. We don't have any plan to look into Huawei's deal at this point," the official said. His minister, Choi Mun-kee, told lawmakers in late October "There's not much the government can do about private companies doing business with Huawei, but there is security concern (involving such deals)."

LG Uplus insisted it would directly manage and operate the system. "Unlike some other foreign countries, we directly manage and control our network," LG Uplus said in a recent statement on the Huawei deal.

"Japan's Softbank Corp also has been using Huawei equipment for more than two years, but their government hasn't raised any issues as they operate the system like we do."

Huawei, the world's biggest supplier of telecoms infrastructure systems, is effectively locked out of large-scale contracts in the US after a House of Representatives intelligence committee recommended that the four largest telecoms networks there should not buy equipment from the company or another Chinese firm, ZTE.

However a Huawei spokesman told the Guardian that reports elsewhere that the company is "withdrawing" from the US network market were untrue. "We have revenues of about a billion dollars there, and we have network equipment contracts with some of the smaller providers," the spokesman said.

Menendez and Feinstein said they were "very interested" to receive the administration officials' assessment of "potential threats and security concerns" about Huawei's involvement, as well as any discussions the US government has had with the South Korean government about the importance of network integrity related to the decision.

A senior administration official declined to discuss details of diplomatic discussions involving Seoul, but added, "We do have concerns about Huawei." The official noted Huawei was excluded in October 2011 from taking part in the building of a US wireless emergency response network due to national security concerns.

• In October 2012 a US report suggested that Huawei and ZTE should be shut out of the US market over concerns for national security