US President Donald Trump has again targeted Iran, saying it is backing "terrorists and militias", and that neighbouring states are aligning with Israel against the "threat" it poses.
Mr Trump arrived in Israel from Saudi Arabia, where he sought to win Arab states' support for fighting extremism.
"The United States and Israel can declare with one voice that Iran must never be allowed to possess a nuclear weapon - never ever - and must cease its deadly funding and training of terrorists and militias.
"And it must cease immediately.
"On those issues there is a strong consensus among the nations of the world, including many in the Muslim world."
He told Israel's Prime Minister, Benjamin Netanyahu, that Iran would never have nuclear weapons.
In a deal with world powers in 2015, Iran accepted curbs on its nuclear programme in return for tangible economic benefits, and the White House confirmed last month that the deal was still holding.
Speaking in Jerusalem, he said Iran had negotiated a "fantastic deal" with his predecessor Barack Obama, winning "a lifeline and prosperity".
But "instead of saying thank you", the Iranians were backing terrorism, he said. In a speech earlier on Monday, he accused Iran of "deadly funding, training and equipping of terrorists and militias".
Mr Trump suggested the Iranians thought they could "do what they want" since negotiating the deal.
Iran's newly re-elected president Hassan Rouhani responded to Mr Trump's comments by saying it was actually his country and its allies that have stood up to terrorists in the Middle East.
"Who fights against terrorism? Syrian, Iraqis, Iranians, Hezbollah and Russia," his translated words said.
"What did the others do, and who can claim that in this region that without Iran the region's stability would be assured?
"Who can claim that without Iran in this region security would be established?"
Mr Netanyahu praised Mr Trump's position.
"I want you to know how much we appreciate the change in American policy on Iran, which you enunciated so clearly just an hour ago.
"I want you to know how much we appreciate your bold decision to act against the use of chemical weapons in Syria and I want to tell you also how much we appreciate the reassertion of American leadership in the Middle East.
Israel PM, Trump push for 'peace'
Mr Netanyahu said he hoped Mr Trump's visit would be a "historic milestone" in achieving regional peace.
In a welcome speech for Mr Trump after he arrived in Tel Aviv from Saudi Arabia, Mr Netanyahu said Israel shared the United States' commitment to peace.
He said Israel's hand was extended in peace to all its neighbours, including the Palestinians.
Earlier, as Air Force One headed to Israel, US Secretary of State Rex Tillerson told reporters on board Mr Trump was prepared to invest his personal efforts into Middle East peace if Israeli and Palestinian leaders were ready to be serious about engaging in the process.
Mr Trump said he was hopeful a lasting peace between Israel and the Palestinians would be possible.
Before his visit he had vowed to do whatever necessary to broker peace between Israel and the Palestinians, dubbing a peace accord "the ultimate deal".
Speaking with Mr Netanyahu after his arrival in Israel, he emphasised the US-Israel allegiance.
"Today we reaffirm the unbreakable bond of friendship between Israel and the United States."
"A friendship built on our shared love of freedom, our shared belief in human dignity and our shared hope in Israel at lasting peace."