The deployment of the most powerful US conventional bomb to have ever been used in combat was a symptom of American concerns over Isis in Afghanistan, security experts have said.
With US and Russian forces playing active roles in both Syria and Iraq, there are fears that war weary Isis fighters could seek refuge in the country.
This is one of the reasons the US may have targeted an affiliate of the Islamist group in the country with a GBU-43 MOAB bomb, known as the Mother of All Bombs, Raffaello Pantucci, Director of International Security Studies Royal United Services Institute (RUSI), told The Independent.
The bomb - the largest non-nuclear weapon ever used in combat by the US military - was dropped on a tunnel complex in Achin district of Nangarhar province, where a US solider on an anti-Isis operation less than a week ago.
It targeted extremist affiliate group Isis in Khorasan Province (Isis-K), a historical area in central Asia which encompasses much of Afghanistan but also parts of eastern Iran, Central Asia and Pakistan.
General John Nicholson, commander of US forces in Afghanistan, said in a statement that the strike was designed to minimise the risk to Afghan and US forces conducting clearing operations in the Achin area "while maximising the destruction" of Isis fighters and facilities.
He said Isis has been using improvised explosive devices, bunkers and tunnels to strengthen its defences, adding: "This is the right munition to reduce these obstacles and maintain the momentum of our offensive against ISIS-K."
This Isis contingent is “quite strong and active in the region the bombing took place,” Mr Pantucci said. “It’s a group the Americans have been trying to confront more aggressively of late. There is increasing concern about what’s happening with Isis in Afghanistan.”
He added that many people treated Isis-K as if they were just another one of the many factions which exist in Afghanistan, a country strongly characterised by its patchwork of warlords and feudal groups.
In addition to native Afghanis, the group is made up of people from other countries in Central Asia and Pakistan.
“What is interesting is that they do seem – from a US-Nato perspective – appear to be gaining strength and a foothold,” Mr Pantucci added. “The concern is, if Isis gets enough of a foothold there, it maybe it becomes a place where you see them leave Raqqa and Mosul for…They do seem to be growing in effect and impact.
“When you compare the range of different Isis affiliates around the world, maybe Isis in Afghanistan have been a bit more interesting and substantial than people first gave it credit for.”
Isis-K has been connected to a number of recent high profile attacks in the country, including on the Supreme Court in Kabul. They have also targeted the Hazara Shia Muslim minority and fought with US and Afghan government forces.
There is therefore a “greater interest” for the US to confront Isis-K, Mr Pantucci said, widening the chance of US fatalities and the potential for such large weapons to be used.
He added that the bomb’s use was a sign of President Trump’s hard line against terrorism, which has also been reflected in a broader mandate for troops in Somalia and Yemen.
“The Trump administration is clearly seeing the hard tools it has at its disposal as tools that it wants to use,” Mr Pantucci said, adding that he expected to see increased drone strikes and special forces operations.
Hans Kristensen, director of the nuclear information project at the Federation of American Scientists, said that the explosive power of even the smallest nuclear weapon in the US. arsenal, the B-61 bomb, is "an order of magnitude" larger than the GBU-43.
He told NBC News that there is a debate inside the defence community on whether to build miniature nuclear weapons.
"We have people arguing for new mini-nukes," he said. "Here you have a case where the U. felt all it needed was a conventional whopper."
"The big unknown with this (GBU-43) bomb is can you get the detonation point close enough to what is in the tunnel," he said. "How deep does it go in? Does this just destroy the entrance?"