The smaller warheads were built in part with US technology gained by espionage, according to US intelligence.

China last month tested its first road mobile intercontinental ballistic missile, which US intelligence analysts believe will have a range of 8,000 kilometers and be targeted primarily against Russia and Asia.

However US officials "expect a test of a longer range mobile ICBM within the next several years; it will be targeted primarily against the United States," according to an unclassified summary of the National Intelligence Estimate of ballistic missile threats to the United States.

"China is developing the JL-2 SLBM (submarine launched ballistic missile), which we expect to be tested within the next decade," it said. "The JL-2 probably will be able to target the United States from launch areas near China."

"By 2015, China will likely have tens of missiles targeted against the United States, having added a few tens of more survivable land- and sea-based mobile missiles with smaller nuclear warheads -- in part influenced by US technology gained through espionage," the report said.

A US congressional probe earlier this year concluded that China stole US nuclear secrets that enabled its scientists to build miniaturize warheads, a requirement for submarine-launched and road mobile nuclear missiles.

The growth of China's nuclear arsenal does not mark a departure from its current posture of maintaining a minimal credible deterrence, said a senior Us intelligence official, who briefed reporters on the intelligence estimate.

"China began to realize that their silo-based systems were vulnerable and so they had to shift to a mobile system," the official said. "The mobile systems are smaller, they are going to have smaller warheads and there has to be more of them ... to hold a significant portion of our population at risk."

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