BEIJING, July 18 -- In a sharp joint statement, China and Russia warned today of "the most grave adverse consequences" if the United States proceeds with plans for a national missile defense system.

The warning was issued at a one-day summit meeting between President Jiang Zemin and his visiting Russian counterpart, Vladimir Putin, in which the two countries also pledged to bring Chinese-Russian ties to a "new" and "hgher" level and focused on what they described as a shared preoccupation with the role of the United States as the world's only superpower.

"The nature of the [American missile defense] plan is to seek unilateral military and security advantages," said a joint communique carried by China's official New China News Agency. "Implementing this plan will have the most grave adverse consequences not only for the security of Russia, China and other countries, but also for the security of the United States and global strategic stability. . . . Therefore China and Russia are firmly opposed to such a system."

The statement also called the 1972 Anti-Ballistic Missile (ABM) Treaty, which bans national missile defenses, "the cornerstone of global strategic stability and international security," and said that "any damage to the ABM will trigger a new arms race."

The communique echoed in stronger and more formal language expressions of opposition by Putin and Jiang relayed by their spokesmen after their first meeting July 5 at a regional security summit in Dushanbe, Tajikistan. It was the latest note in a chorus of complaints arising from Russia and China -- in addition to other countries, including some U.S. allies -- seeking to dissuade President Clinton from going ahead with construction of a limited national missile defense system. Clinton is expected to make his decision this fall.

In a display of Russian support for China's concerns over Taiwan, the communique also warned against deployment of any "nonstrategic missile defense system" that would undermine "security interests of other countries." In particular, it said, "incorporating Taiwan in any foreign missile defense system in any way is unacceptable and will seriously undermine regional stability."

China has voiced worry that providing such a theater missile defense system to Taiwan would undercut the threat of its missiles arrayed in southern China. This, Beijing has warned, could embolden the island to declare independence and would constitute interference in what China regards as a domestic struggle to reincorporate a wayward province.

Moscow's opposition to missile defenses, although often voiced, has been tempered by suggestions from senior Russian leaders, including Putin himself, that Russia might agree to modify the ABM treaty or even consider cooperating with the United States on a limited defense system for Europe. It was unclear in today's communique whether Russia's position has really hardened on these issues or whether, while in China, Putin sought to please his hosts with tough language.

In addition to the particularity of China's concern over theater defenses for Taiwan, Russia has far less to fear from national defenses than does China.

U.S. officials say the national system would be designed to defend against a small-scale attack by such hostile states as North Norea or Iraq. With its arsenal of strategic warheads still numbering in the thousands, they contend, Moscow would retain a credible deterrent threat. China, however, is thought to posses a strategic force of fewer than two dozen missiles, small enough to be compromised by a missile defense system.

This fear underlay the communique's charge that, despite explanations to the contrary, the United States is seeking "unilateral military and security advantages" with its plans for national missile defense.

The White House did not comment directly, but spokesman Joe Lockhart said, "I think President Putin has made his views well-known on this issue. He's discussed this with the president while we were in Russia."

John D. Holum, the acting undersecretary of state for arms control and international security affairs, told reporters that the Chinese and Russian objections were "not unexpected" and reiterated that a U.S. national missile defense system would not be directed at Russia or China.

Holum said that despite the diplomatic costs underscored today in Beijing, national missile defense "is likely to be essential as threats rise from states of concern."

A Western diplomat in Beijing said the Chinese-Russian statement could be intended to capitalize on the failure of a U.S. test earlier this month, the second failure of three such tests.

"Why not add a little gasoline to the fire? With people now wondering whether the system can even work, it's a perfect time to provide critics with more proof that it's a bad idea," the diplomat said.

In addition to formalizing their mutual suspicions of the United States, the two sides sought to stimulate a trade relationship that, while showing signs of improvement, has never met its potential. Bilateral trade in the first half of 2000 totaled $3.56 billion, according to Chinese statistics. While that marked a sharp rise compared to last year, it also means there is little chance of reaching the official target of $20 billion.

Russia is also a major supplier of arms to China. Deputy Prime Minister Ilya Klebanov, who accompanied Putin to Beijing, said earlier this month that sales to China are set to double in value in the next two years. Russia already sells China more than $1 billion a year in armaments.

Staff writer Steven Mufson in Washington contributed to this report.