BEIJING, June 24 -- China has begun negotiations with Russia to buy eight more submarines in a $1.6 billion deal that will significantly boost its ability to blockade Taiwan and challenge U.S. naval supremacy in nearby seas, Western and Russian sources said.

Four Russian producers are bidding to build the diesel-powered Project 636 Kilo-class vessels, which will be equipped with Klub long-range, anti-ship missile systems, defense experts said.

China has already purchased four Kilo-class subs from Russia, including two Project 636 models. The deal for additional submarines is part of a $4 billion weapons package that Russia has committed to provide China over the next four to five years. Included in the package are two more Sovremenny-class destroyers, adding to a pair China has already received, a new batch of S300 PMU2 anti-aircraft missiles and 40 Su-30MKK fighter-bombers.

The $4 billion sale cements Russia's place as China's biggest military trading partner, far ahead of Israel and such former Soviet states as Ukraine. It also cements China's place as the world's biggest weapons importer, underscoring its race with Taiwan for military supremacy across the Taiwan Strait.

China became the world's biggest importer of weapons in 2000, according to the Stockholm International Peace Research Institute. It maintained the No. 1 position last year, mostly through purchases of ships and combat aircraft worth close to $3 billion, more than twice any other buyer's acquisitions.

The United States is Taiwan's main military supplier. In April 2001, the Bush administration approved a multibillion dollar package that included eight diesel submarines, 30 AH-64D Apache Longbow attack helicopters, 12 P-3C submarine-hunting aircraft, four Kidd-class destroyers, long-range radar systems and Patriot III missiles. The U.S. submarine deal is uncertain, however, because the United States no longer makes or designs diesel-powered subs and two nations that do, Germany and the Netherlands, have refused to allow the United States to use their designs or manufacturers.

The Chinese submarine deal will "very significantly enhance [the Chinese] navy's ability to influence events in the East China Sea," said Bernard Cole, an expert on the Chinese navy at the National War College in Washington, "first, by enforcing a blockade against Taiwan, if Beijing adopts that course of action, and also by posing a serious problem for opposing naval forces attempting to operate in the area."

The deal reflects China's double-barreled military modernization strategy. On one hand, the strategy seeks to enable its army to recover Taiwan by force, if necessary. On the other, it wants to deter any intervention by the United States, which has committed itself to Taiwan's defense under the vaguely worded Taiwan Relations Act.

Two U.S. aircraft carrier battle groups faced down Chinese threats to the island in 1996 after China fired missiles across the strait during training exercises. Assuming China could detect the U.S. carriers in the future, such a response would be riskier once the new submarines are operational, defense experts said.

The deal, first reported on June 7 by the Kanwa Intelligence Review, a publication based in Canada and focused on the Chinese military, has prompted competition among Russian manufacturers to win the contract, although China's desire to have swift delivery means that work will occur at several plants simultaneously.

Russian press reports have identified the contenders as the Admiralteyskie Verfi shipbuilding plant based in St. Petersburg, the Komsomolsk-on-Amur shipbuilding plant, the Krasnoye Sormovo shipbuilding plant in Nizhny Novgorod and the Sevmash shipbuilding plant based in Severodvinsk.

China's own submarine manufacturing program has encountered difficulty, especially a multibillion dollar program to develop the Song class guided-missile submarine. China tried to develop the Song to replace its 1962-vintage Romeo-class attack submarines.

According to Jane's Defense Weekly, the first Song, built with assistance from Israel and others, started sea trials in 1995, but proved a failure. A second substantially modified Song began sea trials in early 2000, but analysts say these are far behind schedule and have yet to be completed.

"If Beijing is going to buy eight additional Kilos, it means that their domestic program to build Songs is, in fact, in trouble, which would certainly not surprise me," Cole said.

A second problem associated with China's purchase of the Kilos concerns its ability to use the submarines properly. China bought four Kilo-class submarines during the 1990s, two of the export version and two of the more capable Project 636 version produced by Russia for its own navy. The Chinese navy has experienced operating problems because of initially inadequate crew training, and more consistently because of certain material problems, such as troublesome batteries.

The new Kilo will be equipped with an anti-ship missile system with a range of 140 miles. But China would need to develop the ability to see "over the horizon" to use the weapons properly, defense experts said. Most submarines can only "see" a few miles without the aid of satellites, other submarines, airplanes or ships.

"China still cannot find ships at sea," a senior U.S. defense official said. "Over-the-horizon targeting escapes them. The United States built an open ocean surveillance capability in the 1960s. China has all the tools to build its own but it has not."