By Samia Nakhoul

DUBAI, March 16 (Reuters) - Gulf Arab states did not ask the U.S. to go to war with Iran, but many are now urging it not to stop short by leaving the Islamic Republic still able to threaten the Gulf's oil lifeline and the economies that depend on it, three Gulf sources told Reuters.

At the same time, these sources and five Western and Arab diplomats said Washington was pressing Gulf states to join the U.S.-Israeli war. According to three of them, President Donald Trump wants to show regional backing for the campaign, to bolster its international legitimacy as well as support at home.

"There is a wide feeling across the Gulf that Iran has crossed every red line with every Gulf country," said Abdulaziz Sager, chairman of the Saudi-based Gulf Research Center and familiar with government thinking.

"At first we defended them and opposed the war," he said. "But once they began directing strikes at us, they became an enemy. There is no other way to classify them."

IRAN ATTACKS THE SIX GULF STATES

Tehran has already demonstrated its reach, attacking airports, ports, oil facilities and commercial hubs in the six Gulf states with missiles and drones while disrupting shipping through the Strait of Hormuz - the artery carrying about a fifth of global oil and underpinning Gulf economies.

The attacks have reinforced Gulf fears that leaving Iran with any significant offensive weaponry or arms manufacturing capacity could embolden it to hold the region's energy lifeline hostage whenever tensions rise.

As the war entered its third week, with U.S. and Israeli airstrikes intensifying and Iran firing at U.S. bases and civilian targets across the Gulf, a Gulf source said the prevailing mood among leaders was unmistakable: that Trump should comprehensively degrade Iran's military capacity.

The alternative, the source said, was living under constant threat. Unless Iran was severely weakened, he said, it would continue to hold the region to ransom.

Predominantly Shi'ite Muslim Iran has often viewed its Sunni Arab Gulf neighbours - close allies of the U.S. that host American military bases - with deep suspicion, even if relations with Qatar and Oman have generally been less fraught.

Over the years, Iran and its regional allies have been accused of attacks on Gulf energy installations, not least a 2019 strike on Saudi Arabia's Abqaiq and Khurais oil facilities - for which Iran denied responsibility - that halved Saudi output and rattled energy markets.

For Gulf leaders, inaction is now the greater risk.

The effect of Iran's attacks this month goes far beyond specific material damage, not only disrupting oil flows but damaging a hard-won image of stability and security that has underpinned Gulf countries' attempts to expand trade and tourism and rely less on fossil fuel exports.

"If the Americans pull out before the task is complete, we'll be left to confront Iran on our own," Sager said.

GULF FEARS OF TRIGGERING WIDER WAR

In response to questions about those concerns, the White House said the U.S. was "crushing (Iran's) ability to shoot these weapons or produce more", and that Trump was "in close contact with our partners in the Middle East".

Of the Gulf countries, only the United Arab Emirates responded. It said that it "does not seek to be drawn into conflicts or escalation" but affirmed its right to "take all necessary measures" to safeguard its sovereignty, security and integrity, and ensure residents' safety.

Sources in the region said unilateral military action by any Gulf state remained off the table because only collective intervention would avoid exposing individual countries to retaliation.

Moreover, consensus is still elusive. The six members of the Gulf Cooperation Council - Bahrain, Kuwait, Qatar, Saudi Arabia, Oman and the UAE - have held just one Zoom call, and no Arab summit has been convened to discuss coordinated action.

Gulf leaders remain deeply fearful of triggering a broader, uncontrollable conflagration.

U.S. Defense Secretary Pete Hegseth said last week that Gulf partners were "stepping up even more" and were willing to "go on the offense" while already working with Washington on collective and integrated air defenses, though he did not specify what else they might do.

A senior UAE official said his country had chosen restraint, after Iran said the U.S. military had used the UAE to strike Kharg Island, home to Iran's main oil export terminal.

Yet Sager said Saudi Arabia, Iran's main rival for regional influence, could be forced to retaliate if Iran crossed red lines, notably with strikes on major oil facilities or desalination plants or causing heavy casualties:

"In that case, Saudi Arabia would have no choice but to intervene."

He said Riyadh would nevertheless try to calibrate any response to avoid further escalation.

GULF'S STRATEGIC DILEMMA

At heart, the Arab Gulf states face a strategic dilemma, said Fawaz Gerges of the London School of Economics: balancing the immediate threat of Iranian attacks against the far greater risk of being drawn into a war led by the U.S. and Israel.

Joining that campaign, he said, would add little to Washington's military superiority while sharply increasing exposure to Iranian reprisals. The result is calculated restraint: defending sovereignty and signalling red lines without entering a war the Gulf countries neither started nor control.

Right now, Iran's leverage is evident. It has effectively been deciding which ships can pass through the strait, something no state in the region considers acceptable.

"Now that Iran has shown it can shut down Hormuz, the Gulf faces a fundamentally different threat," said Bernard Haykel, professor of Near Eastern studies at Princeton University. "If it's not addressed, this danger will be long-term."

Trump on Sunday called - with little initial success - for a coalition of nations to help reopen the waterway.

Haykel argued that, while the global economy depends on Gulf oil and gas, most of it flows east to China, Japan and other Asian economies, meaning that they, too, must shoulder responsibility.

"China helped secure maritime routes off Somalia; it may be willing to step in here too," Haykel said.

(Additional reporting by Alexander Cornwell in Jerualem and Parisa Hafezi in Dubai, writing by Samia Nakhoul; Editing by Kevin Liffey)