Jerusalem (CNN) The United States will exert "maximum pressure" to prevent Iran from developing its nuclear program, US national security adviser John Bolton said during a press conference in Jerusalem Wednesday.

Bolton's visit to the region came amid mounting tensions between US and Iran over sanctions, and fears over escalating violence in Syria.

US President Donald Trump announced moves to pull out of the Iran deal in May, abandoning the multinational agreement that had been struck to curb Iran's nuclear program.

The Trump administration began reimposing sanctions earlier this month, with more due to take effect in November against the Iranian oil industry, which accounts for a fifth of the country's GDP.

On Wednesday, Bolton defended Trump's decision to reimpose sanctions, calling Iran "the central banker of international terrorism since 1979."

He added that the deal gave the regime "new assets it could use for its nuclear weapons program, for its ballistic program, for its terrorist activities."

"What we want is massive change in the regime's behavior," Bolton said. But he added: "Regime change in Iran is not American policy."

Earlier this week, in an exclusive interview with CNN, Iranian Foreign Mohammad Javad Zarif accused the US of having an "addiction to sanctions," and deploying them as a weapon that had been proven not to work.

"We felt that the United States had learned that at least as far as Iran is concerned, sanctions do produce economic hardship but do not produce the political outcomes that they intended them to produce, and I thought that the Americans had learned that lesson. Unfortunately I was wrong," Zarif said.

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Bolton issues warning

Addressing Syria, Bolton acknowledged the Assad regime's plan to retake Idlib, one of the last rebel-held enclaves in the country.

He said the US was "concerned" about the possible use of chemical and biological weapons "once again."

"Let there be no confusion, if the Syrian regime uses chemical weapons we will respond," he said.

In April, the US, UK and France launched strikes against targets at three sites in Syria, in retaliation for an alleged chemical weapons attack on civilians in the Damascus enclave of Douma. Damascus denied using such weapons.

Asked in an interview with Reuters if there was an understanding with Russia about any plans by the Syrian regime to launch a full-scale offensive in Idlib, Bolton said there was not.

Bolton also said the Trump administration's intention was to remove Iranian forces from Syria completely, a goal echoed by the Israeli government. Bolton said he would discuss the possibility of Iranian withdrawal with his Russian counterpart, Nikolai Patrushev, when the two meet on Thursday in Geneva.

Bolton said the removal of Iranian forces was also a Russian interest, though he added Russia can't go it alone.

"Perhaps US-Russia joint efforts might be sufficient (to remove Iran)," said Bolton at the press conference. "I'm not sure."

No timing for peace deal

Bolton also stood by the Trump administration's decision to move its embassy from Tel Aviv to Jerusalem as the "right thing to do."

"It was a very positive step forward, not only for Israel, but for the Palestinians as well," he added.

US opened its new embassy in Tel Aviv in May, despite international criticism and fierce objections from the Palestinian Authority. Following the announcement, the Palestinian Authority froze relations with the Trump administration and has vowed to automatically reject any peace plan put forward by the US.

But that hasn't deterred Trump.

At a rally in West Virginia Wednesday, the US President said that Palestinians would "get something very good" in return for the embassy move, but Bolton wouldn't give any specifics on what that may be.

"As a dealmaker, as a bargainer, he would expect, I would expect, you would expect the Palestinians would say, 'OK, we didn't get that one, now we expect something else.'"

Asked about the timing of the Israeli-Palestinian peace plan promised by the US administration, Bolton only said that it was still being formulated and no decisions have been made about its release.

But Bolton took a swipe at the Palestinian Authority when he was asked about a potential ceasefire agreement between Israel and Hamas in Gaza.

"If Hamas cared more about the people of the Gaza Strip than their political priorities, we wouldn't have a lot of these troubles," said Bolton, "and I think it's a sad outcome for the Palestinian people that all they've got now is a choice between Hamas and the Palestinian Authority."