PITTSBURGH (AP) _ A New York investment group offered to buy Allegheny International Inc. for $683.8 million, slightly less than the $700 million it bid in July for the consumer products maker.

Japonica Partners, in a letter to Allegheny International's board of directors released Wednesday after the stock market closed, said it would consider paying $700 million for the company if it is allowed to review certain undisclosed financial information.

The lower offer reflects Japonica's belief that the company's value has deteriorated by $100 million since the beginning of the year, Japonica said.

Allegheny International's board had not responded to Japonica by last night, a Japonica spokeswoman said. Tim O-Brien, a spokesman for Allegheny, said the company was reviewing the offer and had no immediate comment.

Meanwhile, Allegheny International, which has been operating under Chapter 11 protection of the federal Bankruptcy Code, today announced that the Wells Fargo Bank has withdrawn $400 million in financing.

The company had been counting on the money to cover some of its claims and continue operations while it develops a reorganization plan to emerge from bankruptcy court protection. Allegheny International said other financing alternatives were being explored.

A disclosure hearing on Allegheny International's most recent plan to reorganize its debts was scheduled for this morning in Pittsburgh. The company has been operating under Chapter 11 since February 1988.

Japonica, which was created by two former vice presidents of the investment bank Goldman Sachs, is best known for an unsuccessful bid earlier this year to acquire CNW Corp., a Chicago-based holding company with railroad interests.

Japonica's proposed offer includes $90 million in equity from Japonica Partners, $70 million in other investor financing, $335.8 million in bank financing, $135 million in ''excess cash,'' and $53 million in debt reinstatement and tax and loan offsets, Japonica said. It contains no high- yield financing, the so-called junk bonds that have been under attack on Wall Street lately.

Japonica said it was confident it could raise the financing.

Proposed investors also include the Steinhardt Group, a New York money management firm that holds a stake in USAir Group, and several ''global institutions,'' Japonica's letter stated.

Steinhardt is also a creditor in Allegheny International's bankruptcy case.

Allegheny International last month filed two alternate plans to emerge from bankruptcy. The company has filed a total of eight plans in its efforts to reorganize but has met consistent opposition from creditors and shareholders. Its debts currently total about $500 million.