The nation's major stock and commodity markets may halt trading for at least 30 minutes if war breaks out in the Persian Gulf to give investors time to react, sources said.

Government and market officials said they expected the exchanges themselves to impose a half-hour trading stoppage if combat begins during regular trading hours in the United States.But the sources, who spoke on condition of anonymity, also said a halt probably would not be necessary if shooting starts overnight - before the U.S. markets open.

A plan agreed to late last week anticipates a temporary halt on the New York and American stock exchanges as well as the over-the-counter stock market run by the National Association of Securities Dealers, the sources said.

A spokesman for at least one exchange, however, said there was no agreement to a specific 30-minute delay.

The possible trading halt was said to be initiated by the exchanges themselves. The Securities and Exchange Commission has authority to suspend trading, with the president's permission, during market emergencies.

Senior market and government officials have been in close contact about the crisis and would likely coordinate action during an emergency, as they did during the stock market mini-crash in October 1989.

An SEC source would say only that there had been "extensive discussions" with the exchanges about war and a thorough review of systems and procedures in the event of extreme trading volatility.

American Stock Exchange spokesman Bob Shabazian said "in no conversations that we took part in was there any agreement on a 30-minute delay."

"The presumption is that you try to keep the markets open and the only way that a delay or halt might be agreed upon would be if news broke and it was not properly disseminated," he said. "The agreement was if and when you had a halt like that, it should be very short."

Officials at the New York Stock Exchange, the nation's largest stock exchange, declined to say what action they would take at the outbreak of hostilities.

NYSE spokesman Richard Torrenzano said consultation with the SEC and other exchanges would continue, "but until you're presented with a set of circumstances it would be inappropriate to speculate on what you might or might not do."

The Chicago Board of Trade and Chicago Mercantile Exchange said they intended to remain open in the event of war, but said they were prepared to halt trading in stock-index futures if the NYSE closes.

Stock-index futures allow investors to bet on the movement of a series of stocks without having to buy the underlying securities.

It was not clear what the nation's markets would do if there was subsequent market-shaking news after a delay.

"How long is this market coalition going to hold? The markets seem to have come to the conclusion that there's going to be this one halt but what happens after that?" one market source said.