China announced tonight that it has successfully test-fired into the East China Sea its first submarine-based ballistic missile.

Foreign military experts said a successful launching of this sort moves China closer to nuclear capability at sea, although it is believed to be still several years away from operating a nuclear fleet.

China has been seeking for years to diversify its limited nuclear deterrent to reduce the likelihood of a Soviet "surgical strike" aimed at knocking out its land-based strategic force.

Only four other countries -- the United States, the Soviet Union, Great Britain and France -- have developed delivery systems for launching missiles from submerged submarines.

China's claim to a successful firing suggests it has reached the level of technology the United States had achieved in the early 1960s with the first generation of Polaris missiles, according to a foreign analyst.

Official news reports did not specify the range of the unarmed missile, but foreign experts assumed it to be of intermediate distance -- about 1,500 miles.

In May 1980, China announced it had successfully fired two intercontinental ballistic missiles with a range of about 6,200 miles from a land-based launching site, presumably in the northwestern part of the country.

Those tests showed China had the capacity to hit targets in European Russia and the western United States.

Capability to launch missiles from submarines would enhance China's nuclear deterrent by giving it more flexible response to an attack, foreign experts said.

Yet, China is still "at least a few years" away from building the submarines, perfecting a launching system and ensuring the accuracy for a credible nuclear force at sea, according to a Western expert.

"This test shows a reasonably advanced state of technology," said an analyst, "but it's just a first step."

According to Peking radio tonight, the missile "flew quite normally," and landed in the anticipated target area, which is northeast of Taiwan in the East China Sea.

A radio commentator called the test firing "important for . . . strengthening national defense" as well as an "embodiment of socialism's excellence."

"Only because of this excellence have we been able to enter the ranks of advanced nations in sophistication of national defense technology while being backward in economic modernization," said the commentator. The radio replayed the final countdown and takeoff.

According to the official New China News Agency, Communist leaders sent a congratulatory telegram to the missile launchers, saying their success "is another victory for the party's policy of independence and self-reliance."