The United States dropped "the mother of all bombs," the largest non-nuclear device it has ever unleashed in combat, on a network of caves and tunnels used by Islamic State in eastern Afghanistan on Thursday, the military said.

President Donald Trump touted the bombing as evidence of a more muscular U.S. foreign policy since he took office in January after eight years of President Barack Obama.

The 21,600 pound (9,797 kg) GBU-43 bomb, which has 11 tons of explosives, was dropped from a MC-130 aircraft in the Achin district of Nangarhar province, close to the border with Pakistan, Pentagon spokesman Adam Stump said.

The GBU-43, also known as the "mother of all bombs," is a GPS-guided munition and was first tested in March 2003. It is regarded as particularly effective against clusters of targets on or just underneath the ground. Other types of bombs can be more effective against deeper, hardened tunnels.

It was the first time the United States has used this size of conventional bomb in a conflict.

Trump described the bombing as a "very successful mission.” It was not immediately clear how much damage the device did.

During last year's presidential election campaign, Trump vowed to give priority to destroying Islamic State, which operates mostly in Syria and Iraq. He flexed U.S. military muscles last week by ordering a cruise missile attack on a Syrian government airbase in retaliation for a poison gas attack.

"If you look at what’s happened over the last eight weeks and compare that really to what’s happened over the last eight years, you’ll see that there’s a tremendous difference," Trump told reporters at the White House on Thursday.

The security situation remains precarious in Afghanistan, with a number of militant groups trying to claim territory more than 15 years after the U.S. invasion which toppled the Taliban government.

So far, Trump has offered little clarity about a broader strategy for Afghanistan, where some 8,400 U.S. troops remain.

LONG AFGHAN WAR

Last week, a U.S. soldier was killed in the same district as where the bomb was dropped while he was conducting operations against Islamic State.

White House spokesman Sean Spicer said the bombing "targeted a system of tunnels and caves that ISIS fighters used to move around freely, making it easier for them to target U.S. military advisers and Afghan forces in the area."

Spicer said the bomb was dropped at around 7 p.m. local time and described it as "a large, powerful and accurately delivered weapon." U.S. forces took "all precautions necessary to prevent civilian casualties and collateral damage," he said.

Afghan soldiers and police, with the aid of thousands of foreign military advisers, are struggling to hold off a resurgent insurgency led by the Taliban, as well as other groups like Islamic State.

The U.S. government's top watchdog on Afghanistan said earlier this year that the Afghan government controls less than 60 percent of the country.

Foreign policy experts said that it appeared the use of a specialized weapon like the GBU-43 had more to do with the type of target -- tunnels -- than the United States sending any message to other countries by using such a powerful weapon.

"This is a very specialized weapon, we don't have very many of them, you can only use them in a very narrow set of circumstances," said Mark Cancian, a senior adviser with the Center for Strategic and International Studies think tank.

Cancian added that while sending a message to Syria or North Korea could have been among the secondary factors considered, they would not have been the main reason for using this type of weapon.

U.S. Senator Jim Inhofe, a member of the Senate Armed Services Committee, said the use of this bomb was a sign that the United States was committed to Afghanistan.

But Congresswoman Barbara Lee, a Democrat who was the only "no" vote for authorization for use of military force in Afghanistan in 2001, said the move was unprecedented and asked for an explanation.

"President Trump owes the American people an explanation about his escalation of military force in Afghanistan and his long-term strategy to defeat ISIS," she said in a statement.

The top U.S. commander in Afghanistan said recently that he needed several thousand more international troops in order to break a stalemate in the long war with Taliban insurgents.

U.S. officials say intelligence suggests Islamic State is based overwhelmingly in Nangarhar and neighbouring Kunar province.

Estimates of its strength in Afghanistan vary. U.S. officials have said they believe the movement has only 700 fighters but Afghan officials estimate it has about 1,500.

The Afghan Taliban, which is trying to overthrow the U.S.-backed government in Kabul, are fiercely opposed to Islamic State and the two group have clashed as they seek to expand territory and influence.