Trump's announcement appeared to be an attempt to bolster Netanyahu ahead of the vote on Tuesday, but it fell short of the kind of grand gesture Trump had offered ahead of Netanyahu's first major election contest this year, when Trump said the United States would recognize Israeli sovereignty over the Golan Heights.

"Politically speaking, this is a pretty weak gesture," said Natan Sachs, director of the Brookings Institution's Center for Middle East Policy. "Compared to recognizing the Golan Heights, this is pretty paltry."

Trump declined to publicly endorse Netanyahu's promise last week to annex parts of the West Bank if he is reelected, telling reporters he had nothing to say on the matter. The president also said he strongly doubts a report last week of Israeli spying on the White House.

Trump said he looks forward to continuing the discussion about a defence pact "when we meet at the United Nations" this month.

It is not clear whether Netanyahu will attend this year's United Nations General Assembly, which falls the week after the vote. If he wins, he is expected to attend and use the session to argue against international negotiation with Iran, something Trump has said he wants to do.

A mutual defence pact would tighten the US-Israeli military alliance against Iran, which Israel considers its greatest enemy. It could formally obligate the United States to come to Israel's defence in the event of an attack, adding an insurance policy to the alliance that would outlast Trump's tenure unless it was revoked.

Such an agreement would also give Netanyahu a positive development to point to amid a disagreement with Trump over the value of new negotiations with Iran over its nuclear program.

But the idea of a pact is not universally popular. Benny Gantz, a former chief of the Israel Defense Forces and Netanyahu's chief electoral rival, warned in a televised address Saturday that a formal security pact might "tie Israel's hands".

Netanyahu thanked Trump for Saturday's announcement, saying in a tweet that Israel "has never had a greater friend in the White House."

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In an interview, Netanyahu said the formal security pact "will be a historical deal," and he took credit for Trump's interest in a military treaty.

"It is another historic agreement, and I have worked for decades to bring such agreements," Netanyahu said. A treaty, which he compared to the NATO alliance, "will bring greater security for the state of Israel."

The prime minister, who has built his second campaign of the year on his friendship with Trump and his commitment to Israeli security, had reportedly been exploring a formal US security agreement for months. Political observers in Jerusalem had speculated that an announcement might come the weekend before the election, when political bombshells are practically a campaign tradition.

Polling in Israel suggests a close race in what amounts to a do-over of the election five months ago in which Netanyahu declared himself the winner but was unable to assemble a coalition government.