China's anti-ship ballistic missile - a long-feared weapon known as the 'carrier killer' - is close to operational, according to a senior US military official. The commander of the US Pacific Command, Admiral Robert Willard, made the remark in Tokyo this week. He said the US would not be deterred from deploying vessels in the region because of the missile. 'To our knowledge, it has undergone repeated tests and it is probably close to being operational,' Willard told Japanese journalists. 'We have not allowed the development of these capabilities and capacities to deter our right to navigate in international waters in areas around China, nor do you want us to. 'The security in the region is dependent on the regional militaries' presence on the water and in the air ... to ensure that the sea lines of communication and air lines of communication are kept safe.' Willard said concern over such a weapon highlighted the need to resume Sino-US military exchanges. His remarks go beyond previous US statements and reports, including the Pentagon's annual report on China's military build-up, issued last week. In March, he told United States congressmen that China was developing and testing the missile, known by its acronym of ASBM. It also comes after mainland media reported a new base in Shaoguan in northern Guangdong being developed by the PLA's Second Artillery ballistic missile brigade - sparking speculation in Washington that the ASBM could be based there, putting the disputed islands of the South China Sea in range. A successful ASBM could have a range of 1,500 kilometres from the mainland coast, putting much of Japan, Taiwan and the 'first island chain' in range - complicating the traditional strategic picture in the western Pacific, particularly war scenarios involving Taiwan. It also comes as PLA generals take a more assertive stance against the presence of US aircraft carriers operating close to the Chinese coast - a right US officials have always insisted upon as part of freedom of passage in international waters. The missile is one of the PLA's most controversial. The US and the former Soviet Union formally pledged never to pursue building such a weapon. Wary of its costs and dangers, Moscow and Washington included an ASBM ban in arms-limitation talks towards the end of the cold war. By firing a ballistic missile - a rocket that would traditionally carry a nuclear warhead toward a city - to strike a ship, China would risk a catastrophic miscalculation by its enemies, who might fear they were under nuclear attack and retaliate in kind. Such a weapon also represents a significant technological challenge for China's military scientists. The plan apparently under development would see a variation of the DF-21D medium-range missile carry an advanced warhead that would break away in the final stages of flight and manoeuvre towards a moving target, such as an aircraft carrier. Andrew Erickson, an associate professor at the China Maritime Studies Institute of the US Naval War College, wrote this week there was now significant open evidence that China had made ASBM development a priority. A fully integrated flight test - something the PLA would not be able to hide - would be needed to give China's generals full confidence in its deterrent qualities and prompt approval of full-scale production, he said. Pentagon officials said the ASBM was now of great concern but the PLA still faced potential roadblocks in integrating the missile with computerised command and control systems. 'They still have a ways to go before they manage to get that integrated, so that they have an operational and effective system,' a Defence Department official said.