By Ahmad Ghaddar and Alex Lawler

LONDON, April 4 (Reuters) - Saudi Arabia's anger at Kazakhstan and other over producing nations was the key driver behind a shock decision by the OPEC+ oil group to open taps on Thursday and might not be reversed even if oil prices fall further, three OPEC+ sources told Reuters.

Saudi Arabia has been an aggressive supporter of production control to balance the market in the last five years as its budget requires oil prices of around $90 per barrel. Thursday's decision represents a major departure from those policies.

For the past several months, Saudi Arabia has been pushing Kazakhstan and Iraq to improve compliance with production cuts, threatening to otherwise start ramping up its oil output.

Instead, Kazakhstan showed record pumping figures month after month as U.S. firms Chevron and Exxon Mobil expanded production at the key field in the country. Iraq was slow to curtail its volumes too.

The warning shot to the cheaters was fired a month ago, when OPEC+ decided to start modest monthly production increases to the tune of 130,000 barrels per day from April against market expectations it would keep output steady.

But as cheating worsened over the past month, Saudi Arabia fired a bombshell by pushing the OPEC+ group to release 411,000 bpd into the market in May, three times more than expected and representing around 0.4% of global supply.

On Friday, oil prices plunged 8% to below $65 per barrel, their lowest since the midst of the coronavirus pandemic in 2021, on U.S. tariffs, China's retaliation to U.S. tariffs and OPEC+ decision to speed up production hikes.

OPEC+ might take no action to reverse steep output hikes even if oil prices drop below $60 per barrel, one of the three OPEC+ sources familiar with Thursday's discussions said. He asked not to be named due to sensitivity of discussions.

A second source familiar with the deliberations on Thursday said the message from Saudi Energy Minister Prince Abdulaziz bin Salman was that members needed to stick to their targets or more production increases would follow.

OPEC+ and the Saudi energy ministry did not immediately respond to a request for comment.

The first source said low prices would hurt nations both inside and outside OPEC+ and hence all producers would take actions one way or another to limit production.

All three sources said they would not call Thursday's production increase a price war yet even though it has revived memories of Saudi Arabia's clash with Russia for market share in 2020 and OPEC's clash with U.S. shale in 2014-2015.

UNPREDICTABLE LIKE TRUMP

Some sources inside the producers' group said the Thursday decision was driven by much wider geopolitical reasons than just a desire to punish cheaters.

Announced on the same day with U.S. tariffs, the oil hike decision might help Saudi Arabia and Russia to foster relations with U.S. President Donald Trump.

Trump has been calling for lower oil prices just as tariffs are expected to accelerate inflation. He also wants to cut Iranian oil exports to zero to accelerate talks over Tehran's nuclear programme.

During the first price war in 2014-2015, OPEC opened its taps wide to slow the growth in U.S. shale. Some U.S. companies have been forced to cut production and even go bust but growth resumed a few years later to reach new record highs.

In 2020, Saudi Arabia clashed with Russia at the peak of the coronavirus pandemic when oil demand collapsed.

It was Trump who pushed both sides to reach a compromise after saying very low oil prices were hurting the U.S. oil industry too and it needed higher prices to survive.

Back in 2020, oil prices fell below zero.

Such risks are very small in 2025 but prices below $50 per barrel might still be detrimental for U.S. shale producers.

"Trump will undoubtedly welcome lower prices. However, we suspect there will be some concern in the boardrooms of U.S. producers as they have to potentially reset expectations about the OPEC's leadership willingness to provide perpetual support to prices," said Helima Croft from the Royal Bank of Canada.

However, Trump's likely actions against Iranian oil exports may withhold over 1 million barrels per day from the market and hence lend support to prices.

One Russian source said the decision on Thursday was partially driven by the desire to appease Trump amid Ukraine's peace talks.

Another sources pointed to Trump's upcoming visit to Saudi Arabia in the next few weeks and a U.S. decision to impose a low 10% tariff on Saudi Arabia and the United Arab Emirates.

Other OPEC+ members such as Iraq and Kazakhstan, which also exports oil to the United States, will face tariffs of 39% and 27% respectively.

OPEC+ key ministers will meet again on Saturday for what sources say shall be a technical meeting and in early May to possibly review production policies.

"OPEC is becoming as unpredictable as Trump," a third OPEC source said.

(Reporting by Alex Lawler, Ahmad Ghaddar and Dmitry Zhdannikov; Writing by Dmitry Zhdannikov; Editing by David Evans)

((Dmitri.Zhdannikov@thomsonreuters.com;))