Ten of OPEC's 13 oil ministers reached tentative agreement tonight to cut their official oil prices, marking the second time this year that the cartel has been forced to adjust for slack consumer demand and an oil glut.

But several ministers and senior delegates said Iran, Algeria and Libya have not approved the planned price reductions of the Organization of Petroleum Exporting Countries.

At a January meeting here, the same three countries refused to approve lower rates designed to bring OPEC prices more in line with free market levels.

The latest cuts will amount to 50 cents a barrel for heavy oil and 20 cents a barrel for medium grades, conference sources said.

Well-placed delegates said the Saudis asked that the price of extra light oils, produced mainly by North African countries, be raised by 15 cents a barrel.

This request was reportedly withdrawn because most members disagreed with any price increase at a time when the market remains so bearish.

The pricing decision, reached after three days of discussion, was still considered insignificant by oil traders and analysts attending the conference.

They said that the cuts left prices for OPEC heavy crude, produced in large quantities by Saudi Arabia and Kuwait, about $1 above recent rates on the free market.

Venezuela, another heavy crude producer, may still proceed with a further price cut on its own because its prime competitor, non-OPEC Mexico, recently reduced prices for similar oils $2 a barrel below those of its Latin American rival.

The conference is expected to finish its work Thursday. OPEC chairman Subroto of Indonesia plans to make a last attempt to arrange a unanimous agreement, but some delegates said they doubted that Subroto would succeed.

Saudi Arabia's oil minister Ahmed Zaki Yamani is also expected to make a firm plea for all cartel members to cease giving secret discounts.

Oil traders say that cheating is so widespread that 80 percent of all OPEC oil is now sold below the cartel's official rates.

Only Saudi Arabia and Kuwait are said to be adhering scrupulously to the posted OPEC prices.