Oil ministers of Algeria and Nigeria today attacked a compromise pricing agreement reached by OPEC amid speculation that leading oil producers outside the organization might see OPEC's failure to adopt a strong, unified position as a signal to lower their own prices.

The disarray in OPEC increased pressure on North Sea producers such as Britain and Norway, and on Mexico, the largest source of oil for the United States, to lower the price of their lighter grades of crude or to increase production.

There was no immediate indication today how those countries would respond to the move by the Organization of Petroleum Exporting Countries. Mexico said recently it would maintain current prices and production levels through January but that it would consider changing them in February unless OPEC reestablished control of the world oil market.

"This was a negative agreement for OPEC, diminishing the organization's credibility," Algerian Oil Minister Belkacem Nabi told a press conference after the cartel agreed last night to narrow differentials among the prices of heavy, medium and light crude oils produced by the 13 member states, benefiting countries such as Nigeria that produce light crude, which has been in less demand.

The decision was seen by oil experts as a stopgap measure designed to shore up OPEC prices against those on the spot market -- where individual cargoes are sold at prices reflecting daily world market conditions rather than based on long-term contracts. The spot market regularly has been undercutting OPEC's official benchmark price of $29 a barrel -- the reference price for Arabian light crude, from which other prices are figured -- during the past two months.

Nigerian Oil Minister Tamunoemi David-West said that he had refused to assent to what he called a "cosmetic decision" by OPEC and that he hoped the cartel would adopt "a more bold price structure" in January. OPEC's differentials committee will meet Jan. 18 to try to put more muscle in the agreement on prices and differentials.

David-West said Nigeria will press for a "bolder step" -- presumably a bigger increase in the price of heavy crudes -- when the differentials committee meets. He pledged that "OPEC will present a united front against North Sea oil producers" Britain and Norway at that time.

OPEC announced yesterday that the ministers, with the exception of Algeria and Nigeria, had agreed that the benchmark price should be held at $29, that heavy crudes should be increased by 50 cents a barrel and medium crudes by 25 cents and that extra-light crudes should be lowered by 25 cents.

In Los Angeles, oil industry analyst Dan Lundberg predicted that the decision to hold the base price at $29 will fail, United Press International reported. "They cannot possibly succeed because they are operating in a vacuum," he said. "If the non-OPEC nations are not also with them, (if) the North Sea nations, the United States and Canada do not go along with this development, it won't succeed."

Demand has risen recently for the heavier crudes, which are refined into a variety of products in new high-technology refineries. Nigeria had been pushing for an increase of between $1 and $1.50 a barrel in heavy crude prices, which would have made its own light crude oil more competitive. Algeria supported this move.

Kuwaiti Oil Minister Ali Khalifa Sabah, who voted for the compromise, told reporters today that with a little more time OPEC could have achieved an agreement on pricing satisfactory to all.

Nabi predicted that "January will be a very foggy month" for OPEC, but he said he hoped that "common sense will have prevailed by March." After the differentials committee meets in January, oil ministers will reconvene in February to try to forge a more permanent arrangement on differentials.

Nabi attacked Nigeria for "breaking ranks" and selling its oil well below the $29 price set in October. "We cannot talk about disruption any more with Nigeria selling at $3 below the OPEC price," he said.

Nigeria had been given a dispensation to sell at $2 below the $29 benchmark price, but market sources have said it consistently has been lopping off another dollar.