Oil ministers from eight member states of the Organization of Petroleum Exporting Countries ended two days of talks here today without agreement on pricing and output levels but called on the other OPEC countries to join in further talks on Monday.

"There is a realization that this is a very important meeting, a unique one in the history of OPEC," Algerian Oil Minister Belkacem Nabi said. OPEC's continued failure to reach a strategy for dealing with the sharply reduced global demand for oil already has strained the cartel's solidarity to the breaking point.

In comments to reporters after today's session, ministers made it clear that no accord has been reached on the key issues of price and production, although differences appear to be narrowing.

"We are moving closer," said Venezuela's Humberto Calderon Berti. "We don't want and don't need a price war."

Of the five OPEC members who did not attend the London meetings, Iran is the most critical. The Iranians repeatedly have rejected calls led by Saudi Arabia to cut oil's present $34 benchmark price, favoring instead a strategy of substantial reductions in output to dry up the present petroleum glut. The Saudis have thus far resisted that approach.

Ministers said they expected Iran to join the talks Monday. Until an Iranian delegation actually is seated, however, that will remain a major uncertainty. The other countries invited to join the London talks are Iraq, Ecuador, Gabon and Qatar.

Informal consultations among the ministers and technical experts are to continue throughout the weekend. Unable in repeated recent efforts to arrive at a viable working arrangement, the ministers are wary now of predicting eventual success.

Oil analysts believe the current prolonged bargaining is based on the hope that if the members are genuinely alarmed at the prospects of a free-for-all on prices and production, then compromises will eventually be found. To give ministers as much leeway as possible, these talks and the ones planned for next week are portrayed as preliminaries to an official OPEC session to be held once a tentative accord is reached.

The most likely outcome is still reckoned to be a cut in the benchmark price to $29 or $30 per barrel and a ceiling on production of 16.5 to 17.5 million barrels a day.

The main problems are in apportioning quotas for each country within that total and in determining prices for higher-grade African oil, which traditionally carries a premium over the base figure.

Nigeria has already broken ranks with its OPEC partners and cut its price to $30 a barrel, designed to make it competitive with similarly priced North Sea oil pumped by non-OPEC producers Britain and Norway. Faced with serious economic problems, the Nigerians are seeking assurances that any new differential would not place it at disadvantage in attracting customers.

To ease its difficulties on prices and production, OPEC has appealed to Britain to align North Sea rates and production figures with the cartel's. But Prime Minister Margaret Thatcher's Conservative government has refused to do so, insisting that the free market alone should determine oil's costs.

Mexico, another non-OPEC producer, has indicated that it would cooperate with the organization.

The secretive nature of the current OPEC talks and the haphazard manner in which tidbits of information are relayed by ministers struggling through a horde of waiting reporters and camera crews make precise characterizations of their progress a problem.

"The key question is how far down the road they've come toward an agreement that will stand up and be respected," one leading London oil analyst said today. As yet, he said, there is no clear picture.

What is certain is that the majority of OPEC members are eager to retain the benefits of a joint pricing and production policy, although they are reluctant to make concessions to one another toward that objective. Moreover, OPEC's ability to control the flow of oil has declined as such major non-OPEC producers as Mexico and Britain have emerged and global consumption has fallen.

United Arab Emirates Minister Mana Said Otaiba warned today that if, for all these reasons, OPEC now proves unable to reach a viable agreement, the Saudis and their Arab partners in the Persian Gulf region will begin slashing prices.

Attending today's session were Saudi Arabia, Venezuela, the Emirates, Nigeria, Algeria, Liberia, Indonesia and Kuwait.