'Trying to understand what the ... anti-ship ballistic missile system is designed for and against, and its relations with other anti-access capabilities ... is very much an issue that we would like to discuss military-to-military with the Chinese,' he said outside the congressional hearings last week.

This 'raises the importance of a continuous military-to-military dialogue', which was suspended after the US announcement of arms sale to Taiwan, he said.

The ASBM represents one of the PLA's most controversial weapons - developing technology that the US and the former Soviet Union pledged never to pursue. Wary of its costs and dangers, Moscow and Washington signed a ban on the development of such a weapon towards the end of the cold war.

By firing a ballistic missile - a rocket that would traditionally carry a nuclear warhead over a city - to strike a single ship, China would risk a catastrophic miscalculation by its enemies, who might fear they were under nuclear attack and therefore retaliate in kind.

The plan apparently under development would see a variant of the DF-21D medium-range missile carry a technologically advanced warhead that would break away in the last stages of flight and manoeuvre itself towards a moving target, such as an aircraft carrier.

Given the immense stresses on a ballistic missile as it re-enters the earth's atmosphere and falls towards earth and the difficulties of manoeuvring its warhead to a moving target, many foreign scientists and analysts believe the technology represents an immense challenge for China.

But if it succeeds, possession of such a missile could dramatically alter the stakes in the event of a conflict between China and the US over Taiwan - a battle in which US aircraft carriers would normally be expected to play a pivotal role. The ASBM can be considered a powerful asymmetric weapon, providing a deterrent against the strengths of a larger opponent.

Foreign and American military analysts are scrutinising Willard's remarks. Many believe China's ASBM development could be reaching its final stages after extensive funding and theoretical work but are unsure precisely what testing is under way.

There is certainly no sign of a fully integrated flight test on a moving target - something that would be very hard to hide. Such tests are widely considered vital before Beijing's political leadership would be fully committed to funding the production of an ASBM and formally including it among its defensive options.

Mainland officials have not commented publicly, but a report carried by several state media websites noted that mainland press was watching US comments closely.

Dr Andrew Erickson, an associate professor at the China Maritime Studies Institute of the US Naval College, said there were a range of possibilities in terms of testing.

'While system components may be tested separately, and on the ground in many cases, a fully integrated flight test is likely to be necessary to give the PLA confidence in approving full-scale production and deploying ASBMs in a full operational state,' he said, speaking in a private capacity.

Noting the potential significance of Willard's remarks, Erickson said a number of factors were converging to the point some form of ASBM flight tests might be important.

He noted China's expanding defence spending, a clear strategic rationale as well its long development of ballistic missile technology dating back to the 1950s. More specifically, he also noted the reported completion of a factory to produce rocket motors for an ASBM and the launch of advanced maritime surveillance satellites - including several Yaogan satellites in recent months.

As well as missile technology, the ASBM would require a state-of-art military communications and radar system, capable of finding and tracking enemy ships far from the Chinese coast as well as controlling the warhead.