Chinese and Taiwanese fighter jets have flown hundreds of sorties over the past three weeks along the center of the narrow strait of water that separates the two sides, in what analysts called the sharpest military escalation of tension in the area in three years.

One U.S. official called the Chinese sorties "saber rattling," but as the jets have flown closer and closer to each other's shores, the Clinton administration has become worried that the show of force could accidentally lead to actual conflict.

A U.S. official said that China, which rarely sends planes over the Taiwan Strait, has flown more than 100 sorties with three different types of aircraft, including advanced Sukhoi 27s recently acquired from Russia. Another senior administration official said that Taiwanese aircraft have flown a similar number of times and ventured over the center line of the strait, which is about 100 miles wide.

Tensions have flared since Taiwan's President Lee Teng-hui infuriated Beijing on July 9 by saying he wanted to establish "special state-to-state" relations with China. Beijing perceived that as a rejection of the "one China" principle that has served for decades as the bedrock of security and a framework for negotiations between the two sides. Though Taiwan is a self-governing island of 23 million, Beijing is set on its eventual reunification with the Chinese mainland.

"We believe both sides would be well-advised to take precautions so as not to have an inadvertent incident that could escalate," said a senior administration official. "If planes fly very near each other fully armed, even if they are not intended to actually engage, they certainly risk that something unintended could happen."

In addition to tension in the sky, there was tension in the waters of the Taiwan Strait. On Saturday, Chinese police seized a Taiwanese cargo ship carrying gasoline and food to troops on Matsu, a Taiwanese island near the Chinese coast. The ship's 10 crew members were detained and accused of smuggling.

Neither the United States, eager to repair relations with China after the accidental U.S. bombing of the Chinese Embassy in Belgrade, nor Taiwan, eager to avoid raising anxiety among Taiwanese or driving down its already slumping stock market, have publicized the flights. Secretary of State Madeleine K. Albright made no mention of the flights in her recent meeting with Chinese Foreign Minister Tang Jiaxuan, but she urged in general terms that both sides exercise restraint.

Though a senior administration official called the fighter jet maneuvers "risky," he said that the situation didn't match the level of tension or intimidation of 1996, when China launched missile tests off the coast of Taiwan and flew 1,400 sorties in the Taiwan Strait area. At that time, the United States sent 16 warships to the coast of Taiwan as a show of support.

In another flexing of military muscle, China yesterday tested a new long-range rocket that analysts say is designed to threaten U.S. forces in Asia and whose development allegedly was aided by U.S. technology.

The firing of the surface-to-surface missile was announced in a one-sentence report by the New China News Agency. The Chinese report did not identify the missile or provide its specifications, but Western military analysts said they believed the missile was the three-stage Dongfeng 31, which has a range of about 5,000 miles, runs on solid fuel, is launched by a mobile launcher and could carry a miniaturized nuclear warhead.

State Department spokesman James P. Rubin said the test firing had been expected for some time. "It's a new missile, but its range is similar to already existing missiles developed by China," Rubin said. "I think there's nothing new about China having medium- and long-range missiles. They've had them for a long, long time."

But Western military analysts said the testing of the DF-31 should provide U.S. intelligence agencies with some insight into how much China gained from its alleged theft of U.S. design secrets for rocket technology and the W-88 miniaturized nuclear warhead.

In May, a special congressional committee, led by Rep. Christopher Cox (R-Calif.), alleged that U.S. technology contributed to the development of the DF-31 and a similar submarine-launched rocket called the Julang 2. A subsequent report by a Senate committee intimated that a U.S. company helped China improve one of its rocket engines, thereby furthering the DF-31 program.

"The American taxpayer deserves royalties from the warhead to the solid-fuel rocket motor," said Richard Fisher, an expert on Chinese security and former Heritage Foundation fellow. "American technology has made the difference in the success of this missile."

China's announcement of the missile test -- like recent announcements about breakthroughs in producing neutron bombs and small, tactical nuclear warheads -- appears to be designed to increase pressure not only on Taiwan but also on the United States, which has said it would come to Taiwan's aid in the event of an attack by China. Western military analysts said the Dongfeng 31 could threaten American military forces in East Asia and Japan.

The Dongfeng 31 is launched from a mobile platform, making it easier to hide. China's current, fixed-place intercontinental ballistic missiles (ICBMs) also take a longer time to ready for firing.

China's military modernization in recent years has focused on the development of an arsenal of missiles -- considered a cheap way to gain geopolitical influence.

"It's poor man's force projection," said David Finkelstein, a former counterintelligence analyst at the Defense Intelligence Agency and now a researcher at the Center for Naval Analyses in Alexandria. "China's missile force is really the one true force-projection capability it has. . . . [T]hose missiles are not to be sneezed at. If I was in Tokyo I wouldn't be smiling."

Finkelstein said that if China conducts more missile tests, it could indicate there were problems with this launch. If the launch did meet specifications, he said, China could be expected to deploy the missile within several years. The Cox report estimated that the DF-31 would join China's 2nd Artillery forces by 2005.

The DF-31 is designed to replace the DF-4, a missile with half the DF-31's range, which was developed in the 1960s to threaten U.S. forces in the Philippines. China is expected to build 10 to 20 DF-31 missiles for deployment beginning in 2000.

Despite Albright's cordial meeting in Singapore with Tang last week, tensions between Washington and Beijing persist.

China's Foreign Ministry today summoned the No. 2 U.S. diplomat in Beijing, James Moriarty, to protest proposed American weapons sales to Taiwan. The Clinton administration notified Congress Friday that it plans to sell Taiwan E-2T early-warning radar aircraft and spare parts for F-16 fighter jets valued at $550 million.

Rubin said in Washington that the weapons were defensive and that the sales were consistent with the ground rules governing military transfers to Taiwan. "It is common, and expected, for China to complain about any transfer of parts or aircraft like this," he said.

Pomfret reported from Beijing, Mufson from Washington.