Jim Michaels, and Tom Vanden Brook

USA TODAY

The U.S. military dropped one of its most powerful bombs — a massive 21,000-pound munition nicknamed the "Mother of All Bombs" — on an Islamic State tunnel complex in Afghanistan on Thursday. It marked the first time it has been used in combat and reflected the growing flexibility of the Pentagon to wage war.

The target was in a remote area where the risk of civilian casualties would be low. Gen. John Nicholson, the top U.S. commander in Afghanistan, said the decision to drop the bomb was made because it was the best fit for the target.

A senior Defense official told USA TODAY that Nicholson had the authority to use the bomb under President Barack Obama as well as President Trump.

The official, who asked not to be named in order to discuss details of the operation, said Trump was told about the bomb's use but his approval was not required, a point Trump noted, as well.

"We have given them total authorization," Trump said. "This was another very, very successful mission."

Trump had criticized Obama's management of the war against the Islamic State, also called ISIS, pledging to ramp up pressure on the terror group and give the military more flexibility to combat them.

Another Obama critic, Armed Services Committee Chairman John McCain, R-Ariz., said Obama micromanaged military decisions from the White House, such as placing strict limits on troop numbers.

The bomb used Thursday is called GBU-43 or MOAB for massive ordnance air blast, and costs $16 million. It explodes in the air, creating pressure that can collapse tunnels and buildings. It was dropped from a U.S. aircraft in Nangahar province in eastern Afghanistan.

The MOAB has been an option for use in an attack on ISIS for some time and was moved to Afghanistan months ago, the Defense official said. The strike was announced because it was the first time the MOAB was used in combat and the size of the blast was certain to raise questions among locals, the official said.

The massive bomb not only is capable of destroying a tunnel complex, but it also has a psychological impact on the militants because of its destructive power. The blast can be heard for miles. The military uses less powerful munitions to hit smaller target and minimize damage to surrounding structures.

“This is the right munition to reduce these obstacles and maintain the momentum of our offensive against (the Islamic State),” Afghanistan commander Nicholson said in statement. “U.S. forces took every precaution to avoid civilian casualties with this strike.”

White House spokesman Sean Spicer said, "The United States takes the fight against ISIS very seriously and in order to defeat the group we must deny them operational space, which we did."

Fighting between ISIS and Afghan forces has intensified recently. A U.S. Special Forces soldier advising the Afghans was killed in the area Saturday, the first U.S. combat death this year.

The Islamic State, which is based in Syria and holds territory in Iraq, has emerged over the past year in Afghanistan as a rival to the Taliban, the main insurgent group since they were ousted in the 2001 U.S.-led invasion.

When asked whether dropping the bomb sends a message to North Korea as it continues to pursue nuclear and other weapons, Trump said, “North Korea is a problem. The problem will be taken care of.”

Contributing: David Jackson