Twelve of the 13 members of OPEC agreed today to accept a monitoring system designed to stop cheating by members of the cartel on production and pricing policies.

If policing and other questions the Organization of Petroleum Exporting Countries faces are not resolved, members expressed fears of an oil price war.

Ecuador and Nigeria held out as the ministerial session of the organization resumed after a five-day recess to allow oil ministers to consult their governments on the plan.

Ecuador, one of the cartel's smaller producers, said it was having problems with the legal aspects of OPEC overseeing its domestic production and pricing practices. But late tonight Ecuador's oil minister, Javier Espinosa Teran, confirmed that his government had given the go-ahead.

Nigerian Oil Minister Tamunoemi David-West did not arrive in time for the opening of the resumed meeting. When he finally showed up, the ministers decided it was too late for substantive discussions on the monitoring plan, and scheduled debate to resume on Friday.

The monitoring plan, which has not been made public, was devised in a pre-Christmas session by a four-man committee headed by Saudi Arabia's oil minister, Ahmed Zaki Yamani. It is designed to ensure that controls on oil production set by OPEC in October are enforced and that the $29 benchmark oil price is not undercut.

Both price and production norms have been widely flouted since they were proposed Oct. 31, with members pumping nearly 1 million barrels a day above their ceiling. Some states that are desperate to sell their oil are offering hidden discounts that undercut the official price by $2 to $3 a barrel.

Oil industry analysts have been skeptical as to whether OPEC can put enough teeth into the proposed policing mechanism to make the norms stick. OPEC is a nonbinding arrangement in which decisions are reached by consensus.

The Nigerian oil minister, a key player who has broken ranks with the cartel before, had been quoted several days earlier as saying that any body set up to police pricing and production "should be very representative." So far the OPEC ministers have not addressed the question of who will serve on the proposed body, where it will be located or what its exact terms of reference will be.

United Arab Emirates Oil Minister Mana Said Otaiba told reporters that if the question of policing and other problems were not resolved at the current meeting, "we will have a price war."