Fear has been gripping Iranians as Israel vows to continue attacking the Islamic Republic over its nuclear program, with many fleeing the bigger cities, including the capital Tehran, in search of safer areas.

"The full strength and might of the U.S. Armed Forces will come down on you at levels never seen before," Trump warned.

Israel and Iran have opened a new chapter in their long history of conflict. Israel launched a major attack with strikes early Friday that set off explosions in the Iranian capital of Tehran.

Under the Islamic Republic that took power in 1979, enmity toward Israel has been a core ideological tenet of Iranian foreign policy and a key driver in its regional policy. Over decades, their rivalry played out mainly through indirect actions by Iran and by covert operations from Israel.

Israel on Sunday hit targets in Tehran, including the Defense Ministry and sites it claims are tied to Iran's nuclear program.

Iranian media reported that Israel struck the South Pars Gas Field in what would be the first attack on an Iranian energy site.

U.S. military leaders and elected officials are grappling with a worsening diplomatic and security crisis in the Middle East.

The retaliatory strikes came on Saturday, a day after Israel killed top Iranian military leaders and scientists and destroyed an aboveground nuclear enrichment plant near Natanz.

Israel's attack on Iran raises questions like whether the US will get involved, how Iran will respond, and if a wider conflict may be triggered.