Iran has launched a wave of missiles at Israel hours after White House officials warned Tehran was “imminently” planning to attack.

Guardian reporters in Jerusalem witnessed dozens of missiles flying overhead towards Israel’s main coastal cities in a huge attack not long after 7.30pm, with the engines of the rockets clearly visible from below.

While some interceptions could be heard over Jerusalem, many of the missiles appeared to continue on towards the coast and central Israel to the sound of distant bombs.

On the edge of the Old City many stopped to watch the missiles flying overhead in what appears to have been an unprecedented attack.

Barely 10 minutes later a second wave of missiles passed over the city, apparently from a different direction, with the bright flashes of interceptions lighting up the sky as loud booms rang out.

Tehran had vowed retaliation against Israel for a series of attacks against Iran and the militias it backs across the Middle East, including Hezbollah.

The White House earlier warned it had “indications that Iran is preparing to imminently launch a ballistic missile attack” against Israel.

“We are actively supporting defensive preparations to defend Israel against this attack,” a senior official said in a statement, warning that such an action “will carry severe consequences for Iran”.

Iran launched a salvo of drones, cruise and ballistic missiles at Israel in April: 170 drones, 30 cruise missiles and 120 ballistic missiles. Most were downed by Israel and its allies before they hit their targets.

The IDF spokesperson, R Adm Daniel Hagari, had said the US had warned Israel about the impending attack and that Israeli forces were at their “highest readiness – offensive and defensive”.

He added: “Iranian fire on the state of Israel will have consequences. We have plans and we have capabilities.”

The attack will increase concerns that the escalating violence in the region could spiral into a war between Israel and Iran.

On Tuesday evening, the US embassy called on all its employees and their families to shelter in place until further notice. “The US embassy in Jerusalem reminds US citizens of the continued need for caution and increased personal security awareness as security incidents, including mortar and rocket fire and unmanned aircraft system intrusions, often take place without warning,” the warning read.

“The security environment remains complex and can change quickly depending on the political situation and recent events,” it continued.

“In response to security incidents and without advance notice, the US embassy may further restrict or prohibit US government employees and their family members from travelling to certain areas of Israel (including the Old City of Jerusalem) and the West Bank.”

Israel began a ground incursion into southern Lebanon, which it has called Operation Northern Arrow, on Monday evening with a barrage of shelling across the border.

The ground incursion marks the first time Israeli troops have launched sustained operations in Lebanon since 2006, when the two countries signed a peace deal that ended a 34-day war between Israel and the Shia militia Hezbollah, which dominates large swaths of southern Lebanon.

In the first concrete demands since the military launched its incursion, Israeli officials on Monday ordered the residents of about 30 villages in southern Lebanon to evacuate. An Israeli military spokesperson told people to evacuate to north of the Awali River, nearly 35 miles (55km) from the blue line between the two countries, as the IDF targeted what it called Hezbollah “attack infrastructure” along the UN boundary.

It is unclear why Israel asked residents of certain towns in the south, and not neighbouring towns, to leave, nor is it clear why they ordered them to go so far north. “Be careful, you are not allowed to go south. Going south could put your life in danger,” the statement added. “We will let you know when it is safe to return home.”

Residents of Burj al-Shemali, a town of about 60,000 people that was included in Israel’s evacuation order, received calls warning them to leave, prompting many to flee, the mayor, Ali Deeb, told the Guardian.

“Some went to Tyre, some went to the Awali River. Others stayed, as they didn’t have a place to go or they didn’t have the money to leave,” he said, adding that “everyone” in the town was scared.

As of midday on Tuesday, the scope of the ground operation remained unclear. Airstrikes against targets in Beirut and shelling in southern Lebanon had continued overnight, and Lebanese emergency workers said they had recovered 25 bodies and rescued 13 wounded people since 8pm on Monday.

Local people told Reuters that at least 600 had sought refuge in a monastery in the town of Rmeish near the blue line as airstrikes continued.

The Lebanese prime minister, Najib Mikati, met UN officials on Tuesday and said it was “one of the most dangerous stages in [Lebanon’s] history”. He claimed that “about 1 million of our people have been displaced due to the devastating war waged by Israel on Lebanon”.

The UK foreign secretary, David Lammy, called for an immediate ceasefire and warned that “none of us want to return to the years in which Israel found itself bogged down in a quagmire in southern Lebanon”.

“None of us want to see a regional war,” Lammy said in remarks broadcast on the BBC. “The price would be huge for the Middle East and it would have a significant effect on the global economy.”

At the same time, US officials have voiced cautious support for the operation, with the defence secretary, Lloyd Austin, agreeing with the Israeli defence minister, Yoav Gallant, “on the necessity of dismantling [Hezbollah’s] attack infrastructure along the border”.

The Israeli offensive has followed a number of Israeli successes against Hezbollah that appear to have emboldened the prime minister, Benjamin Netanyahu, to move against the Iran-backed organisation despite considerable diplomatic efforts to avert an escalation in the war.

The Hezbollah leader, Hassan Nasrallah, was killed by an Israeli strike on Beirut on Friday, dealing a heavy blow to the militant group and raising fears that conflict could spread across the Middle East.

That followed two weeks of strikes that began with the explosion of pagers and walkie-talkies belonging to Hezbollah members that killed dozens of people and injured thousands more. Israel has since continued to pound Beirut and has also launched strikes on Yemen and Syria, saying it was launching attacks against Iranian-backed militias across the Middle East.