SINGAPORE, JULY 30 -- Twelve leading Pacific trading nations, including the United States, expressed "grave concern" today at the lack of progress in the General Agreement on Tariffs and Trade talks and warned that a breakdown of the negotiations might lead to a collapse of the world trading system.

The countries meeting here are members of Asia-Pacific Economic Cooperation (APEC), an organization that was formed last year in an effort to draw the nations of the Pacific closer together at a time when other parts of the world appear to be forming trade blocs. In addition to the United States and Canada, members include the six ASEAN nations -- Thailand, Singapore, Malaysia, Brunei, Indonesia and the Philippines -- and Japan, South Korea, Australia and New Zealand. ASEAN is an acronym for Association of Southeast Asian Nations.

The APEC nations account for 46 percent of world economic output and one-third of the world's trading volume. U.S. trade across the Pacific, now nearly $300 billion, is about 50 percent more than its trans-Atlantic trade.

Officials of the 12 countries, in their second meeting, said today that it was vital to quickly revive the stalled GATT talks to strengthen and expand the rules of international trade.

Without a successful finish to the GATT talks by a December deadline, officials said, individual countries may feel free to construct new trade barriers and other protectionist measures. "I wouldn't exactly call it the law of the jungle," a senior Canadian official said, "but it would be a major step backward."

In a public statement, the 12 countries were careful not to assess blame for the lack of progress at the GATT talks. But in background briefings, most officials made clear that they see the principal culprit as the 12-nation European Community, which is set to move in 1992 to a common market that will maintain its system of export subsidies for farm products.

This second APEC meeting was intended to be a largely consultative session on world economic trends and on whether to expand the group's membership to include China, Hong Kong and Taiwan. However, at times the meeting turned into a forum for bashing the European Community for its reluctance to drop agricultural trade barriers.

"There certainly has been a strong desire to send a message to the European Community that as EC '92 integration proceeds, that it should proceed in a way that makes the European Community outward-looking" and not a "source of restrictive markets," a senior State Department official said after the first day's session. "The Community's behavior in dealing with agricultural products is not encouraging."

Brig. Gen. Lee Hsien Loong, Singapore's trade and industry minister, was asked at a press conference to identify the party holding up the trade negotiations. He replied by quoting Machiavelli: "The people who are absent are always in the wrong." The European Community is not represented at this meeting.

The Canadian diplomat, in summing up the tenor of today's talks, said, "The great debate on agriculture is the EC on one side with the United States as the protagonist."

The United States maintains its own system of agricultural subsidies to American farmers, the State Department official said, but "we're willing to make those history if other countries are willing to do the same."

Officials here said that when the world was threatened with a similar trading war in the past, the United States, the European nations and Japan -- representing the world's largest economies -- could come together and settle their differences relatively quickly.

Now, however, with the rise of other economies, "there are a lot of other players involved," the State Department official said. "The complexity of the negotiation is multiple," he said. "What that means is {that} we can't wait until the last minute to do a deal."

In a statement here, APEC said that to overcome the impasse, all trading nations "must be prepared urgently to undertake commitments and political decisions to overcome the core difficulties" now threatening the GATT talks.

In a separate statement, ASEANmade an even stronger appeal for urgent action, voicing fears that a total breakdown of the talks might lead the more developed countries to seek arrangements among themselves. "We are concerned that deals will be struck among the principals . . . and developing countries will be left out in the cold," ASEAN said in a statement.