Lebanon’s state news agency said there were blasts in the southern suburbs of Beirut and several other areas. Hezbollah's al-Manar TV also said many pagers had exploded, without identifying those hurt.

Videos and photos on social media appeared to show wounded men sitting or lying on floors and others being rushed to hospitals. Unconfirmed CCTV footage showed blasts in shops.

A Hezbollah official told Reuters news agency it constituted the "biggest security breach yet" since hostilities with Israel escalated 11 months ago in parallel with the Gaza war.

There was no immediate comment from the Israeli military.

However, the events come hours after Israel’s security cabinet made the safe return of 60,000 residents displaced in the north by Hezbollah attacks an official war goal.

"The security cabinet has updated the objectives of the war to include the following: Returning the residents of the north securely to their homes," the prime minister's office said. "Israel will continue to act to implement this objective.”

On Monday, Israeli defence minister Yoav Gallant said the only way to return Israel's northern residents was through "military action", during a meeting with US envoy Amos Hochstein.

“The possibility for an agreement is running out as Hezbollah continues to ‘tie itself’ to Hamas, and refuses to end the conflict,” a statement from his office said.

Israel has repeatedly warned it could launch a military operation to drive Hezbollah away from the border.

There have been almost daily exchanges of fire across the frontier since the day after the start of the war between Israel and Hamas in Gaza on 7 October.

Hezbollah has said it is acting in support of the Palestinian group. Both are backed by Iran and proscribed as terrorist organisations by Israel, the UK and other countries.

Since October, at least 589 people have been killed - the vast majority of them Hezbollah fighters - according to Lebanon's health ministry.

On the Israeli side, 25 civilians and 21 members of security forces have been killed, the Israeli government says.

Iran’s ambassador to Lebanon was among 2,800 other people who were wounded by the simultaneous blasts in Beirut and several other regions.

Hezbollah, which is backed by Iran, said the pagers belonged “to employees of various Hezbollah units and institutions” and that at least two members were among the dead.

The group blamed Israel for what it called “this criminal aggression” and vowed that it would get “just retribution”. The Israeli military declined to comment.

Israel has been exchanging fire with Hezbollah since last October in parallel with the Gaza war, raising fears of a wider regional conflict.

The UN's spokesman said the developments were "extremely concerning, especially given that this is taking place within a context that is extremely volatile".

Hezbollah relies heavily on pagers for communications. The group has warned its members to stop using mobile phones because they could be hacked or tracked by Israeli security forces.

According to Hezbollah, an unspecified number of pagers exploded in multiple areas of Lebanon at around 15:30 local time (12:30 GMT).

One CCTV video showed an explosion in a man’s bag or pocket at a supermarket. He is then seen falling backwards to the ground and crying out in pain as other shoppers run for cover.

"In all my life I've never seen someone walking on the street... and then explode," Musa, a resident of a southern Beirut suburb, told AFP news agency.

"My wife and I were going to the doctor. I found people lying on the ground in front of me," he said. "People didn't know what was happening."

Lebanese Health Minister Firass Abiad told a news conference that casualties were brought to hospitals across the country, including those in southern Beirut suburb of Dahiyeh, the southern port city of Tyre, and in the Bekaa Valley.

He said most had wounds to the face, hands and stomach, and that about 200 were in a critical condition that required surgery or admission to intensive care.

Iranian state TV reported that Iran’s ambassador in Beirut, Mojtaba Amani, suffered "superficial" injuries in one of the explosions.

A source close to Hezbollah told AFP that two of those killed by the pager explosions were the sons of two Hezbollah MPs, Ali Ammar and Hassan Fadlallah. They also identified the dead child as the 10-year-old daughter of a Hezbollah member in the Bekaa Valley.

Fourteen people were also wounded by exploding pagers in neighbouring Syria, where Hezbollah is fighting alongside government forces in the country's civil war, according to the UK-based Syrian Observatory for Human Rights.

“We hold the Israeli enemy fully responsible for this criminal aggression,” Hezbollah said in a statement on Tuesday evening.

“This treacherous and criminal enemy will certainly get his just retribution on this sinful aggression from where it counts and from where it does not count,” it added.

Lebanese Prime Minister Najib Mikati also blamed Israel for the explosions, saying that they represented a “serious violation of Lebanese sovereignty and a crime by all standards”.

Iranian Foreign Minister Abbas Araghchi said he told his Lebanese counterpart that he "strongly condemned Israeli terrorism".

The US has denied any involvement in the incident and urged Iran not to heighten tensions.

Hezbollah did not say what it believed had caused the pagers to explode.

The Wall Street Journal cited a source as saying the affected devices were from a new shipment that Hezbollah had received in recent days. A Hezbollah official also told the newspaper some people had felt the pagers heat up before the blasts.

Overheated lithium-ion batteries can catch fire, but experts said hacking into the pagers and making them overheat would not usually cause such explosions.

A former British Army munitions expert, who asked not to be named, told the BBC the pagers would have likely been packed with between 10g and 20g of military-grade high explosive, hidden inside a fake electronic component.

Once armed by a signal, called an alphanumeric text message, the next person to use the device would have triggered the explosive, the expert said.

Lina Khatib, a Middle East analyst at the UK-based Chatham House think tank, told the BBC: "Israel has been engaging in cyber operations against Hezbollah for several months, but this security breach is the largest in scale.”

Nicholas Blanford, a Beirut-based senior fellow of the American think tank the Atlantic Council, said: “Israel in one fell swoop has rendered combat ineffective hundreds if not thousands of Hezbollah fighters, in some cases permanently.”

“More senior field officers may not have been affected because they simply do not carry electronic communications devices, relying on messengers. But this is a big blow.”

He warned that Hezbollah's leaders would now “face extreme pressure from the ranks and supporters to retaliate heavily”, describing it as "the most dangerous moment" in the Hezbollah-Israel conflict since October.

A statement put out by the Israeli military on Tuesday evening did not comment on the pager explosions, but said the chief of staff Lt Gen Herzi Halevi had held a situational assessment with commanders "focusing on readiness in both offence and defence in all arenas".

It also said there was no change in defensive guidelines to the Israeli public but asked them to remain alert and vigilant.

On Tuesday morning, Israel's security cabinet made the safe return of 60,000 residents displaced in the north by Hezbollah attacks an official goal of the Gaza war.

Israel Defence Minister Yoav Gallant said during a meeting with US envoy Amos Hochstein on Monday that the only way to return northern residents was through "military action".

“The possibility for an agreement is running out as Hezbollah continues to ‘tie itself’ to Hamas, and refuses to end the conflict,” a statement from his office said.

There have been almost daily exchanges of fire across the Israel-Lebanon border since the day after the start of the war between Israel and Hamas in Gaza on 7 October.

Hezbollah has said it is acting in support of the Palestinian group. Both are backed by Iran and proscribed as terrorist organisations by Israel, the UK and other countries.

Since October, at least 589 people have been killed in Lebanon, the vast majority of them Hezbollah fighters, according to the Lebanese health ministry.

On the Israeli side, 25 civilians and 21 members of security forces have been killed, the Israeli government says.