The Islamic State (ISIS/ISIL) in Iraq and rebels in Syria are using tunnel bombs as a potent new weapon, an upgrade to an ancient tactic, according to a Pentagon organization.

“Updating an ancient tactic, Islamic State militants — as well as rebels in Syria — are digging virtually undetectable tunnels, then planting bombs to blow up buildings and other targets,” reports Defense One, quoting JIEDDO (Joint IED Defeat Organization), the Pentagon arm focused on defeating the use of improvised explosive devices (IEDs) as a weapon.

Defense One learned from Pentagon officials and documents that several dozen tunnel bombs have been used by rebels in Syria while ISIS detonated them last week to capture Ramadi, the capital of Anbar, Iraq’s largest province.

The concept behind detonating a tunnel bomb is reportedly quite simple: dig long enough to reach your intended target, plant explosives, and hit the detonator.

“This below the surface attack is particularly destructive to buildings and is appearing increasingly in Afghanistan, Iraq and Syria,” said JIEDDO at a recent briefing, according to Defense One.

“The use of tunnels for IEDs and other purposes will continue to provide a low risk strategic advantage to extremist organizations and therefore requires continued development efforts and fielding of effective mitigation techniques,” it added.

Syrian dictator Bashar al-Assad has designated all groups attempting to topple him, which include ISIS and al-Qaeda affiliate Nusra Front, as rebels.

ISIS in Iraq and rebels in Syria have detonated at least 45 tunnel bombs in the past two years, JIEDDO said.

Although most have been in Syria, U.S. officials told Defense One that ISIS is building a network of tunnels, as well as bunkers and trenches, in Iraq.

“In Syria, rebels have used tunnels bombs to attack government forces under the control of Bashar al-Assad,” reports Defense One. “Many of these tunnels were dug with hand tools to avoid detection.”

“In Iraq, ISIS used tunnel attacks to devastating effect in their assault on Ramadi. On March 11, ISIS forces detonated a tunnel bomb under an Iraqi army headquarters, killing an estimated 22 people,” it adds. “The blast consumed seven tons of explosives in an 800-foot long tunnel that took two months to dig, according to the JIEDDO briefing. On March 15, a second tunnel bomb was used to attack Iraqi Security Forces. The city fell two months later.”

During the briefing, JIEDDO noted that ISIS frequently disseminates videos on social media showing the use of tunnel bombs on its targets.

“As part of an information operations campaign, these attacks are documented and widely proliferated via social media which increases the likelihood of migration to other conflict areas or adoption by other extremist organizations on a worldwide basis,” reportedly said JIEDDO.

The Pentagon organization revealed that tunnel bombs are being used to target military checkpoints, buildings, and other protected establishments.

It can take less than 30 days to dig a short tunnel, while longer ones (no more than 1 mile in length) can take up to nine months to complete, said JIEDDO.

Defense One notes that tunnels have been weaponized by Iran proxy Hezbollah and Hamas in Gaza as a means to smuggle weapons and attack Israel.

“Now their use is spreading, and extending to direct attacks,” explains the article.

“Beyond bombs, ISIS is believed to be using tunnels to move weapons and avoid detection by American and ally fighter jets and drones. (ISIS may even be exploiting Saddam Hussein’s own tunnel network, which is thought to stretch for 60 miles between palaces, military strongholds, and houses,” it adds. “During the U.S. invasion in 2003, Saddam’s forces used these tunnels to move weapons and as hideouts.)”