The batteries of the walkie-talkies used by Lebanese armed group Hezbollah that blew up this week were laced with a highly explosive compound known as PETN, a Lebanese source familiar with the device's components told Reuters.

The way the explosive material was integrated into the battery pack made it extremely difficult to detect, the source said.

Hundreds of walkie-talkies used by the group exploded on Wednesday, a day after thousands of Hezbollah's pagers detonated across the group's strongholds in Lebanon.

Pictures of the walkie-talkies that had exploded showed labels reading "ICOM" and "made in Japan". Icom (6820.T), opens new tab has said it halted production a decade ago of the radio models identified in the attack, and that most of those still on sale were counterfeit.

Yoshiki Enomoto, the general manager of Icom's security and trade division, told Reuters it was possible that an older Icom device had been modified to make a bomb.

It would be difficult to insert an explosive device into the main compartment of the walkie-talkie because its electronics are tightly packed, so it was more likely to have been in the detachable battery pack, Enomoto told the Japanese broadcaster Fuji TV.

The Lebanese source said explosions had occurred even in cases where the battery pack was separated from the rest of the device.

A Lebanese security source had earlier told Reuters that the pagers had been implanted with explosives that were difficult to detect. Another security source told Reuters that up to three grams (0.11 ounce) of explosives had been hidden in the new pagers, apparently months before the blasts.