One of the sources said the explosion could have been caused by an Israeli air strike on Lebanon, while another said an Israeli drone may have been downed.
The sources refused to give further details, with one saying that a “proper investigation” was still to be undertaken into the blast. It was not clear if there were casualties.
An Israeli military spokeswoman declined to comment.
Israel has carried out several air strikes in neighbouring Syria since the start of the four-year civil war there, mostly destroying weaponry that Israeli officials said were destined for Lebanon’s Shi’ite militia Hezbollah, Israel’s long-time foe.
Hezbollah and Israel fought a 34-day war in 2006 in which hundreds of people were killed.
Earlier this year an Israeli helicopter strike in Syria killed several Hezbollah members and an Iranian general. The group hit back days later with a rocket attack that killed two Israeli soldiers on the frontier with Lebanon.
Observers and Lebanese residents have sometimes seen surveillance planes over Lebanon before such strikes.
The Hezbollah-run al-Manar news channel reported the explosion in Bekaa but did not give more details.
Update: according to preliminary information the blast was caused by the downing of a Hezbollah reconnaissance drone by an Israeli jet .
The Lebanese army rushed to scene in an area between Saghbin and Jabal al-Barouk.
Update: Conflicting reports
A Lebanese security source told Reuters that the cause of the blast “is still not 100 percent clear but most probably it was an Israeli air strike to destroy its downed drone.”