They pledged unity and opposition to Iran as diplomatic calls intensify to end a new Israeli offensive in Gaza against Iran-backed Hamas fighters and blessed a European initiative to restore United Nations sanctions on Tehran over its nuclear activities.
"(Iran) chanted, 'Death to America, Death to Israel.' To achieve Death to America, they have to achieve first death to Israel, because we are America's front line here," Netanyahu said.
"They began to race to the bomb ... and so they began to work secretly to activate their weaponization team," Netanyahu said without citing any publicly available evidence. "We knew that if we didn't act within a year they'd have one atomic bomb, possibly two."
Israel launched a 12-day war in June which killed Iranian nuclear scientists along with hundreds of military personnel and civilians, capped off with US bombings of three key nuclear sites.
31 Israeli civilians and an off-duty soldier were killed in Iranian counterattacks.
'Change course'
The joint appearance came as the US-Israeli posture on a post-war Iran remained unclear. US President Donald Trump said the attacks had 'obliterated' Iran's nuclear program and has been ambivalent about the need for further diplomacy.
European powers have urged renewed talks to end the nuclear standoff once and for all, while Israel continues to moot attacks on its foe while Iran remains defiant.
Rubio said the threat from Iran extended beyond Israel to the Persian Gulf and beyond, adding that missiles Tehran sought to build could have reached Europe.
He added that if the Islamic Republic does not "change course," the administration will continue to apply "maximum pressure" sanctions.
Britain, France and Germany last month triggered the so-called snapback mechanism enshrined in a partly lapsed 2015 international nuclear deal with Iran to restore international sanctions within 30 days over Iran's alleged non-compliance.
Iran denies seeking a nuclear bomb or threatening the West and labels the sanctions move diplomatic blackmail aimed at its sovereignty. Tehran has threatened to halt cooperation with UN nuclear inspectors advanced by an interim deal with the inspectors last week if the sanctions move goes ahead.
Rubio said he supported the European initiative "100%" and that Iran had obviously violated its nuclear obligations.
Israel is continuing a push to overrun Gaza City to crush Hamas fighters there and release hostages - a campaign which has drawn criticism from the United Nations and rights groups but is supported by Washington.
Netanyahu said that military successes over Iran did not void the need for an Israeli victory in Gaza.
"We acted like somebody who has two lumps of cancer. One lump is the atomic cancer, the specter of atomic bombs, and the second one is the specter of 20,000 ballistic missiles, one ton ballistic missiles that fall Mach six to Mach eight right from the sky," he said.
"We were fighting not only our enemies. We were fighting your enemies, and now we're circling back to Gaza to finish the job where it all began."