"We are going to make a very strong call in the meeting that the countries that are over-producing cut," said Venezuelan Oil Minister Rafael Ramirez.

Saudi Arabia has lifted output to 10 million barrels daily, its highest in decades, to help nurse sickly global economic growth in what Saudi Oil Minister Ali al-Naimi has called a "type of stimulus" for the economy.

That has taken supply from the Organization of the Petroleum Exporting Countries to 31.6 million barrels a day in May, an OPEC report estimated on Tuesday, well above the official 30-million-bpd target it set in December.

Oil has fallen from near $128 a barrel for Brent crude in March to trade at less than $98 on Tuesday.

"We think we need to keep the ceiling on production of 30 million that was agreed at our last meeting in December," said Ramirez.

"The first and most important issue is we agree to stick to the 30 million," agreed Iran's OPEC governor Mohammad Ali Khatibi.

Its oil output curbed by U.S. and European sanctions, Iran has grown increasingly irritated as its regional Middle East rival Saudi Arabia lifts supply at its expense.

Iranian output is at a 2-year low just above 3 million bpd.

Riyadh on Monday risked inflaming relations with Tehran by suggesting OPEC might need to lift its output target to match demand in the second half of the year.

But it appeared to back away from that position on Tuesday, making it most likely the group will leave supply policy unchanged.

Saudi Oil Minister Ali al-Naimi told reporters he was "happy with the way things are."

OPEC's in-house experts say demand for fuel is cooling.

"Signs appear to be showing that the global economy is slowing further," the group's Vienna-based secretariat said in a monthly report.

"The second half of the year could see a further easing in fundamentals, despite seasonally higher demand."

The report said demand would average 30.74 million barrels per day in the second half of the year.

That would imply OPEC needs to slice about a million barrels daily from existing output levels if it wants to prevent inventories building further in the second half of the year.