UK to almost double troops in Afghanistan after Trump NATO criticism
An extra 440 non-combat serviceman from the Welsh Guards will be deployed to the war-torn country by February 2019, bringing the total to 1,090.
They will help "bring the stability and security that the Afghan people deserve," May said, ahead of national elections in Afghanistan in October.
The commitment comes after US President Donald Trump lashed out at NATO allies over America's defence spending.
Trump set the stage for clashes at the NATO summit by writing to around a dozen allies to berate them for lagging on a 2014 pledge to try to spend two percent of GDP on defence by 2024.
Speaking to a cheering crowd at a rally this week, Trump told them that the US would no longer be "the schmucks paying for the whole thing [Sic]."
"The US is spending many times more than any other country in order to protect them," Trump tweeted on Wednesday.
Britain is one of only five countries in the 28-member alliance that currently meets the NATO targets of spending 2 percent of GDP on defence, and May will call on allies to match the contribution.
At the height of the war, NATO had more than 130,000 troops from 50 nations in Afghanistan. As of last month, the number stood at 16,000 from 39 countries, focusing on training and assisting the army and police.
Washington's war in Afghanistan is still ongoing after 17 years of unrest, making it the longest conflict in US history. The past few years have seen a particular escalation of the offensive against the Taliban, which has led to an upshoot in violence and casualties.
In 2017, more than 23,000 people died due to the conflict, including some 10,000 security forces, 10,000 Taliban members, and an estimated 3,438 civilians.