The UK ended combat operations in the country in 2014. (Picture: MoD/Crown Copyright).
The UK will send 440 more troops to Afghanistan taking the number of British personnel in the country to more than a thousand.
It comes amid pressure from US President Donald Trump for European allies to contribute more to their collective defence.
Ahead of the latest NATO Summit, Prime Minister Theresa May was keen to announce that as well a supplying additional troops for Afghanistan, the UK was committed to NATO and is just one of the five alliance members to meet the target of spending 2% of GDP on defence. She said:
"The alliance can rely on the UK to lead by example, not just in meeting the 2% pledge but by contributing our cutting-edge capabilities to operations around the world.
"In committing additional troops to the Train Advise Assist operation in Afghanistan we have underlined once again that when NATO calls the UK is among the first to answer."
The extra troops will be from the Welsh Guards who will deployed in a non-combat role, helping to provide security for international advisers in Kabul, as well as mentoring Afghan forces.
The personnel will start being deployed in August with a second contingent following in February 2019.
It will take the number of UK military personnel in Afghanistan to 1,100.
Yesterday, President Trump tweeted that the US spent "many times more" on their defence than any other alliance member and that "Nato countries must pay MORE, the United States must pay LESS. Very Unfair!".
President Trump is expected to continue that pressure and demand into the meeting.
Following the NATO summit, President Trump will then make his first visit as President to Britain before going on to hold talks with Russian President Vladimir Putin in the Finnish capital Helsinki.
'Afghanistan Deployment A Sizable Increase'
By James Hirst, Forces News correspondent
This is a sizeable increase - 440 is about another two-thirds on top of the 660 already there.
It remains a non-combat mission to support Afghan forces.
But this is at least the third time troop numbers have been increased since the UK and NATO’s combat missions ended four years ago.
UK troop numbers will now be more than twice what they were at the end of 2014.
It’s because Afghan forces have not progressed in the way that was hoped; their numbers have fallen in part because they’ve taken significant casualties in the ongoing fight with the Taliban.
We’ve seen the continuing fragile security situation with a recent series of spectacular and deadly attacks.
That’s why NATO wants to, once again, increase the size of its resolute support mission at this summit from its current number of 16,000 - the extra 440 UK troops are part of that plan.
But there’s a new question mark, with reports Donald Trump is about to start a wholesale review of America’s strategy in Afghanistan.
Around half of this UK uplift will arrive next month, coming initially from the Welsh Guards. The other half will be in place by February next year.
They will be working, like those already there, in and around the capital Kabul.
The UK leads the Kabul Security Force which is an enabler for NATO advisors, and mentors Afghan forces securing the capital.
This is a particularly crucial time for those Afghan Forces with elections in October which could be a major target for terror attacks.