US President Joe Biden and Israeli Prime Minister Yair Lapid signed a joint pledge on Thursday to deny Iran nuclear arms, a show of unity by allies long divided over diplomacy with Tehran.

The undertaking, part of a “Jerusalem Declaration” crowning Biden’s first visit to Israel as president, came a day after he told a local TV station that he was open to “last resort” use of force against Iran.

“We will not allow Iran to acquire a nuclear weapon,” Biden told a news conference following the signing of the declaration.

Washington and Israel have separately made veiled statements about possible preemptive war with Iran - which denies seeking nuclear arms - for years. Whether they have the capabilities or will to deliver on this has been subject to debate, however.

Thursday’s statement reaffirmed US support for Israel’s regional military edge and ability “to defend itself by itself.”

“The United States stresses that integral to this pledge is the commitment never to allow Iran to acquire a nuclear weapon, and that it is prepared to use all elements of its national power to ensure that outcome,” the statement added.

Lapid cast this posture as a way of averting open conflict.

“The only way to stop a nuclear Iran is if Iran knows the free world will use force,” he said after the signing ceremony.

Speaking alongside him, Biden described preventing a nuclear Iran as “a vital security interest for Israel and the United States and, I would add, for the rest of the world as well.”

There was no immediate comment from Tehran.

In 2015, Iran signed an international deal capping its nuclear projects with bomb-making potential. In 2018, then-US President Donald Trump quit the pact, deeming it insufficient, a withdrawal welcomed by Israel.

Iran has since ramped up some nuclear activities, putting a ticking clock on world powers’ bid to return to a deal in Vienna talks.

Israel now says it would support a new deal with tougher provisions. Iran has balked at submitting to further curbs.

Eyes on Saudi Arabia

Beyond enhancing the allies’ sense of deterrence and mutual commitment, the statement’s power-projection may also offer Biden a boost when he continues on to Saudi Arabia on Friday. Riyadh has its own Iran worries, and Biden hopes to parlay that into an Saudi-Israeli rapprochement under US auspices.

Earlier on Thursday, Biden told reporters he and Lapid had discussed “how important it was, from my perspective, for Israel to be totally integrated into the region”. Lapid, in turn, deemed Biden’s Saudi trip “extremely important to Israel.”

Some Israeli as well as Gulf Arab officials believe the nuclear deal’s sanctions relief would provide Iran with far more money to support proxy forces in Lebanon, Syria, Yemen and Iraq. They are also skeptical about whether the Biden administration will do much to counter Iran’s regional activities.

A US official, asked if Thursday’s declaration was about buying some time with Israel as Washington pursues negotiations with Iran, said: “If Iran wants to sign the deal that has been negotiated in Vienna, we have made very clear we’re prepared to do that. And, at the same time, if they’re not, we will continue to increase our sanctions pressure, we will continue to increase Iran’s diplomatic isolation.”

A senior Israeli official described the threat of military action as a means of avoiding war.

“(It) is a guarantee that the diplomatic, economic and legal efforts against Iran will be effective,” defense Ministry director-general Amir Eshel told Israel’s Kan radio. “Iran has shown everyone that when it is pressed hard it knows how to stop and change its ways.”

The Jerusalem Declaration further committed the United States and Israel to cooperating on defense projects such a laser interceptors, as well as on civilian technologies.

The US was open to future defense grants to Israel, the statement said, reaffirming Washington’s interest in reviving talks on an Israeli-Palestinian two-state solution.