Joseph E. Seagram & Sons raised the stakes today in a multibillion-dollar bidding war with E.I. du Pont de Nemours & Co. for control of Conoco Inc., the nation's ninth biggest oil company.

Du Pont's offer of $6.9 billion in cash and stock for Conoco last week would be the largest amount ever paid for a company. Most analysts thought it would pre-empt Seagram's first bid.

But Seagram, best known for its Canadian distillery operation, has long been in the oil and gas business. Earlier this year it lost an attempt to buy a big U.S. natural resources company, and Seagram's announcement today indicates the company thinks that control of an oil company like Conoco will be a good investment.

Seagram said today it would buy at least 51 percent (44.35 million shares) of Conoco for $85 a share. On June 25 Seagram said it would buy 35 million shares of oil giant for $73 a share.

Du Pont offered to pay $87.50 a share for Conoco stock, but is willing to buy only 34.44 million shares for cash. It said it would trade 1.6 shares of its stock for each of the remaining 52 million shares of Conoco.

Although Seagram's latest offer is $2.50 a share less than Du Pont bid, Seagram is willing to buy more Conoco stock for cash. Thus, the amount Seagram is willing to spend in cash is at least $3.77 billion, compared with Du Pont's $3 billion cash and $3.9 billion stock offer.

Whether the bigger cash outlay by Seagram will appear more lucrative to Conoco shareholders is uncertain.

Du Pont, which borrowed $3 billion from its bankers to make the Conoco bid, could sweeten the cash portion of its proposal. Similarly Conoco, which reportedly arranged a $3 billion loan of its own last week, could make an attempt to buy some of its own shares to thwart the Seagram bid if Du Pont backs off. Du Pont also might decide to go wity its original offer.

The formal registration statement Du Pont filed last Tuesday with the Securities and Exchange Commission before making an actual offer to Conoco shareholders has not cleared the regulatory agency.

Conoco and Seagram have battled each other bitterly for nearly a month. Conoco, which last month lost its Canadian oil and gas holdings to Dome Pretroleum, spurned a "friendly" bid from Seagram to buy 25 percent of the company's stock.

Conoco then tried to merge with Cities Service Co. to create a firm too big for Seagram to tackle. Cities Service backed out of the deal when Seagram made its tender offer at $73 a share.

Conoco then found a "white knight" suitor in Du Pont, which sees in Conoco a captive source of petroleum for its giant petrochemical operations. If the Conoco-Du Pont merger succeeds, it would create the seventh-biggest industrial concern in the United States, with annual sales of more than $32 billion.

Seagram said today that its offer for Conoco will expire on July 24, but it would make changes in its tender offer if another offer comes on the market (presumbly that of Du Pont, which is not yet formal). It reserved the right to buy more than 51 percent of Conoco's stock.