Washington's plan to expand an advanced missile-defense system in Asia is directed at Pyongyang, not Beijing, the US State Department said on Thursday.

However, military experts in both the United States and China questioned the US' intentions, saying the expensive system, which is well beyond Pyongyang's military capability, is actually "looking at China".

The Chinese military also called for the US to handle anti-missile issues with great discretion and avoid "letting its own state security take priority over other countries' national security".

The Wall Street Journal's front-page story on Thursday talked about Pentagon plans to put a second X-Band early-warning radar in southern Japan to complement one that has been in the country's north since 2006.

It said the US military has also been evaluating sites in Southeast Asia for a third X-Band radar to create an arc that would allow the US and its regional allies to "more accurately track any ballistic missiles launched from (the Democratic People's Republic of Korea), as well as from parts of China".

Some US defense officials said the Philippines, which is at odds with China on territorial issues, is a potential site for the third radar.

Martin Dempsey, chairman of the US Joint Chiefs of Staff, confirmed later on Thursday that the US is in discussions with Japan on the issue.

"It's certainly a topic of conversation because missile defense is important to both of our nations," Dempsey said at the start of a meeting with his visiting Japanese counterpart, Shigeru Iwasaki, at the Pentagon.

Victoria Nuland, a spokesperson for the US State Department, said on Thursday the missile defense work is not directed at China.

"They are designed against a missile threat" from the DPRK, she said, adding that the system is a defensive one, and that Washington has told Beijing about it.

"We do have regular conversations with China ... We are quite open and transparent about what it is that we're doing and why," she said at the daily news briefing.

The People's Liberation Army's deputy chief of general staff, General Cai Yingting, started a US visit this week, which follows one in May by Defense Minister Liang Guanglie. It is not clear whether the anti-missile system is being discussed during Cai's visit.

However, China's Ministry of National Defense on Thursday said in a statement to the Wall Street Journal that "China has always believed that anti-missile issues should be handled with great discretion, from the perspective of protecting global strategic stability and promoting strategic mutual trust among all countries.

"We advocate that all parties fully respect and be mindful of the security concerns of one another and try to realize overall safety through mutual benefit and win-win efforts, while avoiding the situation in which one country tries to let its own state security take priority over other countries' national security."

Beijing objected to the first X-Band deployment in Japan in 2006. Moscow also voiced similar concerns about the system in Europe and the Middle East.

"The focus of our rhetoric is (the DPRK)," Steven Hildreth, a missile-defense expert with the Congressional Research Service, an advisory arm of the US Congress, told the Wall Street Journal.

In April, Pyongyang launched a rocket that blew up less than two minutes into its flight.

"The reality is that we're also looking longer-term at the elephant in the room, which is China," Hildreth said.

Jonathan Pollack, Asia-Pacific security expert at the Brookings Institution, said that in theory, this new radar expansion will have the capabilities to go well beyond the defense of Japan.

In order to prevent a major potential escalation of that kind of competition in East Asia, "for this reason alone, I believe there is a pressing priority for the next president of the United States, either President (Barack) Obama or Governor (Mitt) Romney, to open a series of discussions with China about issues of missile defense," he said.

The US presidential election will be in early November, and the winner will be sworn into office on Jan 20.

Li Qinggong, deputy secretary of the China Council for National Security Policy Studies, said the radar arc can look at missiles launched from coastal area of China.

"The early-warning radar is the key part of an anti-missile system. It can detect a launched missile, as well as determine its trajectory and model.

"It will be like killing a fly with a bazooka if it is used to contain Pyongyang. I believe it is mainly aimed at detecting China's missiles."

The Wall Street Journal report said Washington is concerned over the growing imbalance of power across the Taiwan Straits.

However, Yin Zhuo, a Beijing-based military expert, said "the US won't spend so much energy on Taiwan, to implement ballistic missiles, interceptor missiles and GPS radars everywhere.

"Just a GPS radar costs more than $1 billion," Yin said, adding that Taiwan is just one of the many considerations in the program.

"And to 'protect' Taiwan is just a move for the US to deal with China, not an ultimate goal."