David Cameron said the country had paid a "very high price" after three soldiers were killed in a roadside bomb attack in Afghanistan but said such the work being done in the region was "vital" to make sure the country "doesn't again become a haven for terrorists".
He was commenting after the Ministry of Defence announced that the soldiers, from The Royal Highland Fusiliers, 2nd Battalion The Royal Regiment of Scotland, died while on routine patrol when an improvised explosive device in the Nahr-e Saraj district of Helmand Province struck their vehicle. Next of kin have been informed.
The deaths take to 444 the number of UK service members who have lost their lives since operations in Afghanistan began in October 2001. Six have now died in 2013.
Mr Cameron made the comments to ITV Daybreak and added: "today our thoughts should be with the families and friends of those that have suffered."
Another six soldiers were also injured in the blast and nine Afghans were killed when the bomb struck the Mastiff, a fortified vehicle used to reduce the number of troops killed and injured by IEDs.
Former Army chief of staff Lord Dannatt told Radio 4's Today programme: "It would seem that this was an extremely large bomb that was so powerful that actually it was able to cause fatalities within the vehicle itself.
"I've not seen a technical report but my understanding in talking to the Ministry of Defence is that in all probability it was a very large device in terms of the amount of explosive and it may well have physically lifted up the vehicle and possibly even turned it over."
Asked if it was harder to justify the deaths given the fact British forces will soon withdraw from Afghanistan, he said: "It certainly makes deaths like these ones more painful as we are close to the end.
"I think again if you look at the history of any campaign, particularly when we have said we are going to leave somewhere, that often those who we are fighting increase their efforts towards the end to try and play up the fact that they have driven us out or to increase their hand in bargaining at the negotiating table subsequently."
The last time so many soldiers were killed in one incident was when six died in March last year when their Warrior armoured vehicle was blown up by a massive improvised explosive device about 25 miles north of the capital of Helmand province, Lashkar Gah.
Corporal Jake Hartley, 20, of 3rd Battalion, the Yorkshire Regiment, was killed alongside Private Anthony Frampton, 20, Private Christopher Kershaw, 19, Private Daniel Wade, 20, and Private Daniel Wilford, 21, all also of 3 Yorks, and Sergeant Nigel Coupe, 33, of 1st Battalion The Duke of Lancaster's Regiment.
The force of the Taliban blast - the deadliest single attack on British forces in Afghanistan since 2001 - turned the Warrior upside down and blew off its gun turret.
Announcing the deaths on Wednesday, Major Richard Morgan, a spokesman for Task Force Helmand, said: "It is with deep sadness that I must confirm that three soldiers from The Royal Highland Fusiliers, 2nd Battalion The Royal Regiment of Scotland, have died after their vehicle was struck by an improvised explosive device in the Nahr-e Saraj district of Helmand Province.
"Their deaths come as a great loss to all those serving in Task Force Helmand.
"Our thoughts and prayers are extended to their family and friends at this difficult time."
The Royal Highland Fusiliers, 2nd Battalion The Royal Regiment of Scotland is an infantry battalion, deployed to Afghanistan in March from their base in Penicuik, Midlothian, near Edinburgh.
A Ministry of Defence spokesman said the incident underlines the continued danger faced by soldiers as they prepare to pull out of Afghanistan in 2014.
"Security in Helmand, where most UK forces are based, is steadily improving with Afghan forces already responsible for the bulk of the province - but the environment in which our troops operate remains risky and dangerous, including the threat of improvised explosive devices and insurgent attack.
"We will continue to do all we can to minimise these risks but they can never be removed entirely."
The attack came on the third day of what the Taliban has called its spring offensive.
Other blasts throughout the country killed nine civilians and a police commander.
In past years, spring has marked a significant upsurge in fighting between the Taliban and Nato forces with their local allies.
The insurgents warned they would infiltrate enemy ranks to conduct "insider attacks" and target military and diplomatic sites with suicide bombers.
This fighting season is a key test, as the international coalition is scheduled to hand over security responsibilities to Afghan forces next year.