A strike by Iran on Israel is a "real and credible" threat, the White House has said, as fears of an imminent attack by Tehran grow.
Tensions are high over the possibility Iran will retaliate for an air strike that killed senior Iranian commanders nearly two weeks ago.
US officials had told CBS News, the OceanNewsUK's US partner, that a major attack on Israel could happen imminently.
Israel has said it is ready "defensively and offensively".
"I can't speak to the size, scale, scope of what that attack might look like," US National Security Council spokesman John Kirby said on Friday. But he added that the Iranian threat was "viable," saying Washington was "watching this very, very closely".
He said: "We are in constant communication with our Israeli counterparts about making sure that they can defend themselves against those kinds of attacks".
Iran backs Hamas, the armed Palestinian group fighting Israel in Gaza, as well as various proxy groups throughout the region, including some - such as Hezbollah in Lebanon - that frequently carry out strikes against the Israelis.
On Friday, Hezbollah said it had launched "dozens" of rockets from Lebanon towards Israel. An Israel Defense Forces (IDF) spokesman said around 40 missiles and two explosive drones had been fired. No casualties were reported and there were no indications of involvement from other actors.
A US official told CBS the barrage of rockets and drones was separate from any expected Iranian attack on Israel.
President Joe Biden has already promised Israel his country's "ironclad" support in the event of an Iranian attack.
The mounting tensions have led countries including the US, UK, India and Australia to warn against travelling to Israel.
The US State Department also barred diplomatic staff and their families in Israel from travelling outside of the cities of Tel Aviv, Jerusalem and Beersheba.
On Friday, Israeli Prime Minister Benjamin Netanyahu met members of his war cabinet amid the warnings.
But some Israelis said they were not worried about a potential Iranian attack.
"We know that we are surrounded by enemies, in the south, in the north, the east and the west," Daniel Kosman said from a market in Jerusalem.
"We are not afraid, I can promise you. Look around: people are going out."
The possibility of an Iranian attack has led to concern and anxiety in Israel. But the government has not issued any new advice to the population on top of existing guidance to stock up on water, food for three days and essential medicine.
Israeli radio, however, reported local authorities had been told to prepare for the possibility of an attack, including by assessing the readiness of public shelters.
Last week, amid the first indications an Iranian response was possible, the Israeli military cancelled home leave for combat troops, bolstered air defences and called up reservists.
Thirteen people were killed in the 1 April missile strike on the Iranian consulate in the Syrian capital Damascus.
They included senior Iranian military leaders, among them Brig Gen Mohammad Reza Zahedi, a senior commander of Iran's elite Quds Force in Syria and Lebanon.
Israel has not commented, but is widely considered to have carried out the attack.
Officials in several countries have been trying to dissuade Iran from launching an attack on Israel, fearing it could spark a wider regional war.
US Secretary of State Antony Blinken has spoken to the foreign ministers of China, Saudi Arabia and Turkey to attempt to convince them to use their influence with Iran.
Mr Netanyahu said his government was prepared "to meet all of the security needs of the State of Israel".
After meeting the commander of US Central Command on Friday, Israeli Defence Minister Yoav Gallant said the bond between the two countries had been strengthened by the threat of an Iranian attack, adding "we will know how to respond".
The current war in Gaza was sparked when Hamas attacked Israeli communities near the Gaza Strip, killing about 1,200 people, most of them civilians, and taking more than 250 hostage. Israel says that of 130 hostages still in Gaza, at least 34 are dead.
More than 33,600 Gazans, the majority of them civilians, have been killed during Israel's retaliatory campaign in Gaza, the Hamas-run health ministry says.
The conflict has also seen Israel trade almost daily fire across its northern border with the Iran-backed Lebanese group Hezbollah, while Iran-backed groups in Iraq and Yemen have attempted to hit Israeli territory as well as US bases in Iraq and Syria.
Yemen's Houthi movement has also attacked shipping in the Red Sea, sinking at least one ship and prompting the US and UK to launch missiles against the group.
