Washington (AFP) - The implementation of the Iran nuclear deal marked a major step forward in Washington's cautious rapprochement with Iran, but will further strain ties with Saudi Arabia.

While President Barack Obama's government insists its goal was simply to halt the spread of atomic weapons, experts detect an effort to bring a new balance to its Middle East relationships.

That bore fruit this week with the quick release of 10 US sailors captured in the Gulf by Iranian forces, the freeing of five US prisoners in Iran and the formal implementation of the accord.

But, inevitably, with Tehran and Riyadh daggers drawn, the thaw in ties with Shiite powerhouse Iran can only feed paranoia in the Sunni Gulf monarchies, traditionally close US allies.

"One of Obama's visions for this region, at least in the Gulf, is equilibrium. He uses that phrase a lot," said Frederic Wehrey of the Carnegie Endowment for International Peace.

- 'Equilibrium shattered' -

Beyond ongoing wars in Syria and Yemen, where Saudi-backed forces are fighting Iran-backed forces, broader regional peace can only come if the two build a working relationship.

But, whatever Washington's ambitions, the rival powers are far from that. In fact, the US outreach to Iran over the nuclear deal has only served to make Saudi Arabia more anxious.

At the New Year, Riyadh executed a Shiite cleric for sedition, provoking protests in Iran that led to the sacking of the Saudi embassy and a breakdown in diplomatic relations.

Wehrey said Obama had hoped that if the rivals could, "if not reach some detente or rapprochement, at least be balanced and to sort of get along" then US focus could turn to Asia.

"Of course that ambition of equilibrium has been shattered," the Oxford University academic added.

Experts predict that after the dust settles following implementation of the Iran deal, Obama will arrange a visit to the Arabian peninsula to mollify skeptical Saudi and Emirati leaders.

And Secretary of State John Kerry pre-empted the signature by flying to London on Thursday for talks with the angry Saudi foreign minister, Adel al-Jubeir.

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Jubeir, a former ambassador to Washington, is used to getting a sympathetic hearing from his ally, but Kerry made it clear going into the meeting that there were issues to discuss.

Kerry did not say so, but officials in Washington have made it clear that the administration was dismayed by Sheikh Nimr al-Nimr's execution, which dramatically worsened the crisis.

"The United States will stand with our allies and friends in the region, and we do. But we also want to see diplomacy work," Kerry told reporters ahead of the talks.

- 'Destruction' -

For his part, Jubeir was keen to remind Kerry how Saudi Arabia had worked closely with its US partner in the past, and had tough words for the Iranian leadership Kerry is courting.

Asked whether he was worried the windfall that Iran is about to enjoy as trade sanctions are lifted, Jubeir told Sky News: "Every country in the world is worried about this."

"Iran's record has been one of war and destruction, terrorism, destabilization, interference in the affairs of other countries," he insisted, speaking after talking with Kerry.

This week, Jubeir could be forgiven for seeking to underline what he sees as the Iranian threat to America: Iranian forces seized two boatloads of US sailors in the Gulf.

Instead, the sailors were released within 16 hours and the State Department chalked up the resolution of the crisis to Kerry's relationship with Iran's foreign minister.

Many in Washington, never mind Riyadh, are furious -- alleging that the White House is soft-pedaling Iranian provocations to protect the Iran nuclear deal, despite the loss of face.

- Junior partner -

"In my opinion what's paramount for Obama is to protect the Iran deal," said Karim Sadjadpour, Iran expert and another senior associate of the Carnegie Endowment.

"You could argue it's not only the top of his agenda for the Middle East but arguably what his administration believes to be the crowning foreign policy achievement."

Before Iran's Islamic revolution, former US presidents like Richard Nixon had a two-pillar strategy of maintaining ties with both the shah's Iran and Saudi Arabia.

After the United States cut its own ties with Tehran in 1979 in the wake of the embassy hostage drama, Riyadh became the preferred partner, and with the oil boom a rich one.

The balance of power has shifted in recent years, however.

Riyadh blames Washington for the rising Iranian influence in post-Saddam Iraq, and is disappointed with the US for not standing by its ousted Egyptian friend Hosni Mubarak and for backing Arab Spring revolts.

"And I think this is the fundamental fear, this sort of strategic jealousy that the US may be tilting back toward some sort of equilibrium with Iran," said Wehrey.

"And this harkens back to the status of Saudi Arabia under the shah, with the Twin Pillars doctrine under Nixon, when the Saudis were the junior partner."