We've heard of and seen terrorist attacks launched on the ground and in the air such as bombings. A war veteran familiar with the workings of terrorist groups, however, warned recently that extremists may use a different tactic to strike a blow on the United States: an attack launched from below the ground.

Dan Gordon, an author and a veteran of six Israeli wars, said the jihadist Islamic State may use an existing underground network of tunnels to sow terror in the US, a CBN News report said.

"The country that is most vulnerable to that attack is my other homeland, the United States of America—far more vulnerable because there are terrorist tunnels that exist already," Gordon said.

The war veteran, who wrote the book "Day of the Dead" focusing on these network of tunnels, said Mexican drug cartels have already previously used these underground passageways to smuggle illegal drugs into the US.

These tunnels—which run from Mexico into California, to Arizona and to Texas—may present an opportunity for an unholy alliance between drug smugglers and terrorist groups, Gordon warned.

"They don't even have to dig them; they just have to rent them from Mexican drug cartels. They have billions of dollars; that's a language all the drug cartels understand," he said.

Gordon pointed out that the Palestinian Islamic organisation Hamas used the same technique to carry out sneak attacks on Israel last summer.

The ISIS may have gotten some tips from Hamas on how to carry out strikes from these underground highways, he warned.

"They watched what Hamas did. They saw why it failed. They know how it can succeed. What people don't realise is the Middle East for ISIS is their R&D lab, their Silicon Valley, to what they want to do in the United States," Gordon said.

The war veteran further said that cities and towns at the southern border, such as San Diego, are the most vulnerable.

"The reason the United States is so vulnerable to that [is] we don't have the surveillance over that border like Israel does. We don't have our most elite military units on constant patrol on the US Mexican border," Gordon explained.