A second deadly wave of unprecedented explosions in the strongholds of Lebanon's Hezbollah left it in disarray on Thursday, hours before a major speech by its leader Hassan Nasrallah.

The latest batch of device explosions killed 20 people and wounded more than 450 others on Wednesday, officials said, stoking fears of a full-blown war with Israel.

The blasts came a day after the simultaneous detonation of pagers used by Hezbollah killed 12 people, including two children, and wounded up to 2,800 others across Lebanon, in an unprecedented attack blamed on Israel.

Walkie-talkies used by its members exploded in the latest blasts at Hezbollah's Beirut stronghold, a source close to the group said, with state media reporting similar detonations in south and east Lebanon.

"The wave of enemy explosions that targeted walkie-talkies... killed 20 people and wounded more than 450," Lebanon's health ministry said in a statement.

There was no comment from Israel, which only hours before Tuesday's explosions had announced it was broadening the aims of its war in Gaza to include its fight against Hamas's ally Hezbollah.

"The centre of gravity is moving northward," Israeli Defence Minister Yoav Gallant said during a visit to an air base on Wednesday.

"We are at the start of a new phase in the war."

Amos Harel of the left-leaning Haaretz newspaper said the pager and walkie-talkie blasts had put "Israel and Hezbollah on the brink of all-out war".

With tensions in the Middle East spiralling, senior diplomats from the United States, Britain, Germany, France and Italy will meet on Thursday in Paris, ahead of a UN Security Council meeting planned for Friday.

US Secretary of State Antony Blinken will join his counterparts in the French capital after discussing the possibility of a Gaza war truce in Cairo.

The White House warned all sides against "an escalation of any kind".

Iran-backed Hezbollah has traded near-daily cross-border fire with Israel since Hamas's October 7 attacks sparked the war in Gaza.

Lebanese Foreign Minister Abdallah Bou Habib warned that the "blatant assault on Lebanon's sovereignty and security" was a dangerous development that could "signal a wider war".

Hezbollah said Israel was "fully responsible for this criminal aggression" and vowed revenge.

Iran's envoy to the UN said the country "reserves the right to take retaliatory measures" after its ambassador in Beirut was wounded.

The influx of so many casualties all at once overwhelmed medics.

At a Beirut hospital, doctor Joelle Khadra said "the injuries were mainly to the eyes and hands, with finger amputations, shrapnel in the eyes -- some people lost their sight."

A doctor at another hospital in the Lebanese capital said he had worked through the night and that the injuries were "out of this world -- never seen anything like it".

A source close to Hezbollah, asking not to be identified, said the pagers were "recently imported" and appeared to have been "sabotaged at source".

After The New York Times reported that the pagers had been ordered from Taiwanese manufacturer Gold Apollo, the company said they had been produced by its Hungarian partner BAC Consulting KFT.

A government spokesman in Budapest said the company was "a trading intermediary, with no manufacturing or operational site in Hungary".

The attack dealt a heavy blow to Hezbollah, which already had concerns about the security of its communications after losing several commanders to targeted strikes in recent months. (AFP)