Israel's Mossad spy agency planted explosives inside 5,000 pagers imported by Lebanese group Hezbollah months before Tuesday's detonations, a senior Lebanese security source and another source told Reuters.

The operation was an unprecedented Hezbollah security breach that saw thousands of pagers detonate across Lebanon, killing nine people and wounding nearly 3,000 others, including the group's fighters and Iran's envoy to Beirut.

The Lebanese security source said the pagers were from Taiwan-based Gold Apollo, but the company said it did not manufacture the devices, but were made by a European firm with the right to use its brand.

Iran-backed Hezbollah has vowed to retaliate against Israel, whose military declined to comment on the blasts.

The plot appears to have been many months in the making, several sources told Reuters.

The senior Lebanese security source said the group had ordered 5,000 beepers from Gold Apollo, which several sources say were brought into the country earlier this year.

Gold Apollo founder Hsu Ching-Kuang said the pagers used in the explosion were made by a company in Europe that had the right to use the Taipei-based firm's brand, the name of which he could not immediately confirm.

"The product was not ours. It was only that it had our brand on it," he told reporters on Wednesday, without naming the company which did make the devices.

The senior Lebanese security source identified a photograph of the model of the pager, an AP924, which like other pagers wirelessly receive and display text messages but cannot make telephone calls.

Hezbollah fighters have been using pagers as a low-tech means of communication in an attempt to evade Israeli location-tracking, two sources familiar with the group's operations told Reuters this year.

But the senior Lebanese source said the devices had been modified by Israel's spy service "at the production level."

"The Mossad injected a board inside of the device that has explosive material that receives a code. It's very hard to detect it through any means. Even with any device or scanner," the source said.

The source said 3,000 of the pagers exploded when a coded message was sent to them, simultaneously activating the explosives.

Another security source told Reuters that up to three grams of explosives were hidden in the new pagers and had gone "undetected" by Hezbollah for months.

Israeli officials did not immediately respond to Reuters requests for comment.

Images of destroyed pagers analysed by Reuters showed a format and stickers on the back that were consistent with pagers made by Gold Apollo.

Hezbollah was reeling from the attack, which left fighters and others bloodied, hospitalised or dead. One Hezbollah official, speaking on condition of anonymity, said the detonation was the group's "biggest security breach" since the Gaza conflict between Israel and Hezbollah ally Hamas erupted on Oct. 7.

"This would easily be the biggest counterintelligence failure that Hezbollah has had in decades," said Jonathan Panikoff, the U.S. government's former deputy national intelligence officer on the Middle East.