An unexpected and still unexplained shortage of Conoco Inc. shares yesterday forced Seagram Co. to extended the deadline for delivering the Conoco stock it purchased in the bidding war for control of the oil company.

The stock was supposed to be delivered to Seagram by midnight Wednesday, but Seagram extended the deadline until 5 p.m. today.

The shortage is believed to have occurred because some investors apparently promised to sell Conoco shares they didn't own, hoping to buy them before the stock had to be delivered.

Securities and Exchange Commission officials said yesterday violations of federal securities laws could be involved in the shortage of shares that occurred after a three-way bidding contest for Conoco.

As is usual in such cases, the SEC refused to confirm or deny that it is investigating the Conoco trading.

SEC officials said it is illegal for investors to offer to sell stock they don't own and illegal to simultaneously accept two offers from competing bidders in a fight for control of a company. The SEC has not yet determined whether that happened.

Agency officials said there could be legitimate reasons why people who offered their Conoco stock to Seagram may not be able to deliver it. Failure to deliver stock as promised is not illegal, but it could result in financial penalties.

In the most costly corporate takeover fight in U.S. history, Seagram, Mobil Corp. and E.I. du Pont de Nemours & Co. all tried to buy Conoco. Du Pont eventually won by offering to trade 1.7 shares of its stock for each share of Conoco, toping Seagram's cash offer of $92.

Seagram now plans to take all the Conoco shares it bought and trade them for Du Pont stock, which will make Seagram the biggest stockholder of Du Pont.

As of yesterday, however, Seagram had not received several thousand shares of Conoco stock from investors who promised to sell it to Seagram. Seagram officials would not say how big the shortfall is, but market sources said it could be as many as 200,000 shares of stock, worth more than $18 million.

The shortage of Conoco shares first showed up in the stock option market, where investors buy and sell "options" giving them the right to purchase stock at some future date at a fixed price.

On Wednesday many investors who held options to buy Conoco stock learned they would not get the shares because of an extraordinary decision made Tuesday by the Options Clearing Corp. The OCC handles the paperwork of stock option trading, collecting shares from sellers and delivering them to buyers.

The clearing house decided to cut short the usual proceedure because it could not get enough Conoco shares to deliver to investors who asked for them. Instead of the stock, the investors will be paid $92 cash, the same price they could have gotten by selling the stock to Seagram.

This was the first time that the clearing house had been forced to interfere in the market and order that options transactions be settled for cash rather than stock.

There was not enough Conoco stock to deliver to investors who held options to buy it because most of the shares already had been sold to Seagram, Du Pont and Mobil.