VIENNA, JUNE 27 -- OPEC oil ministers have agreed on a moderate production increase for the rest of the year meant to keep the price of oil at $18 dollars a barrel.

The slight production hike became possible, according to a communique issued at the end of a three-day meeting here, because of the success of the organization's production limits since last December.

The production ceiling was raised from 15.8 million barrels per day in the second quarter of this year to 16.6 million barrels per day for the last half of 1987.

Iraq did not join the agreement. It is the only member of the Organization of Petroleum Exporting Countries that refuses to recognize the pricing and quota system approved last December in Geneva.

The meeting appointed the chief delegates of Nigeria, Indonesia and Venezuela to visit member countries, starting with Iraq, in order to "motivate them to comply with production quotas."

With the completion of a new pipeline to Turkey at the end of the year, Iraq could sharply increase its oil deliveries, and this could pose a threat to the organization's carefully adjusted oil supply system and price level.

Divisive issues such as the Iran-Iraq war and the expanding U.S. role in the Persian Gulf as well as the reflagging of Kuwaiti ships carefully were left from the agenda.