NEW YORK, JAN. 11 -- If war erupts in the Persian Gulf while U.S. financial markets are open, stock and commodity exchanges will likely close for 30 minutes to give investors time to digest the information, Wall Street and government sources said today.

Bank and securities regulators, exchange officials and regulators from foreign countries also have added direct telephone and telefax lines -- and secured the lines from possible terrorist attack -- to ensure that communication isn't disrupted in a crisis, according to securities industry and congressional sources.

"There have been a lot of 'what if' scenarios," a source said.

In general, officials from the various stock and commodities exchanges, and the regulators who oversee their operations, believe that existing regulations and procedures are sufficient to handle the heavy volume and sharp swings in prices that would be anticipated once news of a shooting war moves over the news tickers. Many of the reforms were adopted in the wake of the stock market plunges in 1987 and 1989.

But nobody is taking any chances.

In meetings all week, the Securities and Exchange Commission and the Commodities Futures Trading Commission have been working with officials from the Treasury and the Federal Reserve Board to identify securities and commodities firms that might be vulnerable to cash shortages from heavy trading following an outbreak of war. Plans are in place for the Fed to make sure that any trading house that needs cash can get it quickly and in sufficient quantities, government sources said.

Securities firms have already bolstered cash reserves in anticipation of possible war, securities industry officials said.

But whether the preparations prove sufficient depends on how investors react to war, and no one can anticipate that accurately, they said.

In one concrete step taken because of the danger of war, the Chicago Mercantile Exchange (CME) today increased its requirements concerning margin -- or the amount of cash put up by buyers -- for future contracts for the Standard & Poor's 500 stock index, Treasury bills, interest rates and foreign exchange.

The action is intended to dampen speculative trading by increasing the amount of cash that a trader must put up to buy a contract. The CME, in a prepared statement, said that it took the measures "in light of the potential for market volatility if hostilities commence."

And in New York today, the FBI and city police briefed 300 companies with an international presence, including banks and Wall Street firms, about how to guard against terrorist attacks, which are considered especially likely in New York or Washington, industry sources said.

Banks worldwide and exchange officials in the United States have been warned to gird for possible terrorist attacks that could range from attempts to sever telephone lines to attempts to blow up computer records.

On Wall Street, it has been widely predicted that news of the outbreak of shooting will within hours send oil prices up the maximum $15 a day limit on the futures market, while the Dow Jones industrial average plummets by between 100 and 200 points.

Both the exchanges and the regulatory agencies believe strongly that every effort should be made to keep the markets open, because any closure would tend to reduce public confidence.

But five well-placed securities industry sources, who spoke on condition that they remain unidentified, said that there was a plan to suspend trading for half an hour at all stock exchanges -- including the New York Stock Exchange, the nation's largest -- and other equities markets to permit news of war or other major events to be widely disseminated if it were considered necessary to prevent investors from being penalized for lack of information.

"If, for example, we suddenly got a flash that said, 'war breaks out,' and the market dropped 50 points in 30 seconds, it would be appropriate" to suspend trading for 30 minutes, a source said.

The purpose would be to avoid "having only those who happened to be watching CNN at that moment to have an advantage over those who were not," according to the source.

The exchanges are counting as well on other reforms adopted after the October 1987 market crash to rein in volatility. If the Dow drops 250 points, for instance, the rules already provide for a one-hour suspension of trading. If, after reopening, the Dow plunges another 150 points -- for a total drop of 400 points on the day -- the NYSE would suspend trading for two hours.

There also are rules to limit the impact of program trading -- which is large-scale, computer-driven sales of stocks -- if the Dow rises or falls by 50 points. Because of a special telephone network set up after the 1987 crash, all the nations' and commodities stock exchanges and the key regulatory agencies can communicate with each other at once.

During the mini-crash of 1989, for example, CFTC Chairman Wendy Gramm was unable to coordinate with her counterpart at the SEC because she simply couldn't get a phone call through. McCartney reported from New York, Day from Washington.