Thousands of people were injured across Lebanon and Syria on Tuesday by an Israeli covert operation that remotely detonated pagers, ABC News sources confirmed.
A source described the attack as a "huge operation" that took between six and 12 months to plan, involving the use of informants and collaborators. Explosives were implanted inside the beepers, the source added.
The pagers each contained about 1 to 2 ounces of explosives, according to a source familiar with the attack.
The attack killed at least 12 civilians -- among them an 8-year-old girl and an 11-year-old boy -- according to Lebanese Health Minister Firass Abiad.
At least 2,800 people were injured, Abiad said. More than 460 people underwent surgery for serious injuries, the minister added. Most victims are suffering from eye and facial injuries, while others suffered injuries to hands and fingers, he said.
The Hezbollah militant group confirmed that 11 of its members were killed on Tuesday, though did not specify the manner of their deaths.
At least 14 people were also injured in targeted attacks on Hezbollah members in Syria, according to the Syrian Ministry of Foreign Affairs.
Hezbollah vowed a "reckoning" for Israel. Leader Hassan Nasrallah is due to speak on the situation on Thursday afternoon.
The pagers began exploding around 3:30 p.m. local time, according to Hezbollah officials. An intelligence source familiar with the situation told ABC News that Israel has long been working to perfect this type of "supply chain interdiction attack."
MORE: Israel behind pager explosions, sources say, as Hezbollah vows 'reckoning'Responding to media reports that the explosives were concealed inside its AR-924 pager model, Taiwan-based beeper maker Gold Apollo told ABC News it was not responsible for the design or manufacture of the item.
"According to the cooperation agreement, we authorize BAC to use our brand trademark for product sales in designated regions, but the design and manufacturing of the products are solely the responsibility of BAC," they said.
ABC News has contacted BAC for comment. The company is based in Budapest, Hungary.
Hungarian authorities have confirmed that the company in question is a trading intermediary, with no manufacturing or operational site in Hungary, according to a Hungarian government spokesman. It has one manager registered at its declared address and the referenced devices have never been in Hungary.
During further investigations, Hungarian national security services are cooperating with all relevant international partner agencies and organizations.
The government added the devices pose no national security risk to Hungary.
The Lebanese Council of Ministers collectively condemned "this criminal Israeli aggression, which constitutes a serious violation of Lebanese sovereignty and a crime by all standards."
It added that "the government immediately began making all necessary contacts with the countries concerned and the United Nations to place it before its responsibilities regarding this continuing crime."
The United Nations special coordinator for Lebanon called the operation an "extremely concerning escalation in what is an already unacceptably volatile context," in a statement released by the U.N. Office of the Spokesperson for the Secretary General.
MORE: Israeli defense officials at odds with Netanyahu over Hamas, Hezbollah cease-fire conditionsThe White House, State Department and Department of Defense all emphasized at briefings that the U.S. was not involved and was not aware of “this incident," specifically referencing the pagers exploding.
Israel indicated to the U.S. that it intended to carry out an operation targeting Hezbollah prior to the pager explosions, but did not share information about the tactics of the plan or its scope, according to one U.S. official.
The U.S. and the European Union have both designated the Hezbollah militant group a foreign terrorist organization.
ABC News' Morgan Winsor, Joe Simonetti, Jordana Miller, Ghazi Balkiz and Helena Skinner contributed to this report.