He told BBC Radio 4's Today programme troops were there to "ensure that Afghanistan can not again become an incubator for terrorism and a launching pad for attacks on us".
"This is about the future of Britain because we know that the borderlands of Afghanistan and Pakistan have been used to launch terrible attacks, not just on the US but on Britain as well," he added.
Conservative leader David Cameron said the "whole country will be shocked by the deaths of so many service personnel".
BBC defence and security correspondent Rob Watson
When British troops were first deployed to southern Afghanistan three years ago the then defence secretary expressed the hope that they would complete their mission without a shot being fired.
It has instead been the most high intensity fighting British troops have faced since the Korean War in the 1950s.
To critics, the ferocity of the fighting is proof of how ill thought out the whole mission has been all along.
Defenders of the operation, however, say it was always bound to be difficult and that the casualties while regrettable have been suffered in a worthwhile and winnable cause.
Certainly the deployment to Afghanistan of around 10% of Britain's army has proved a real strain on manpower, equipment and finances.
For now at least though, Britain remains firmly committed to staying the course.
He said their bravery was "outstanding" and they were "fighting to prevent terrorism on the streets of Britain".
But he said the government "must explain its strategy in Afghanistan" and deliver key equipment to troops.
"It is a scandal that our forces still lack the helicopters they desperately require to move around in Helmand," he said.
"Promises of more helicopters in the future are not enough. More helicopters are needed today. More helicopters would save lives."
Britain's Chief of Defence Staff, Sir Jock Stirrup, said Britain had "taken some very sad casualties over the last several days".
He warned of more casualties to come but insisted the Taliban were "losing" in Afghanistan.
The five soldiers were killed while on the same foot patrol near Sangin, Helmand province.
Lieutenant Colonel Nick Richardson, spokesman for Task Force Helmand, said: "While there are no words to ease their loss, our heartfelt sympathies go to their families, friends and fellow soldiers at this very difficult time: their deaths were not in vain."
Earlier on Friday, it was confirmed a British soldier from the 2nd Royal Tank Regiment died near Nad Ali in Helmand. That news came just hours after the deaths of another two soldiers were announced.
Those two soldiers died on Thursday evening. One was from 4th Battalion The Rifles and he was killed in a blast while on foot patrol near Nad Ali.
The second, from Princess of Wales's Royal Regiment, attached to 1st Battalion Welsh Guards, was killed during an engagement with insurgent forces near Lashkar Gah.
Fifteen soldiers have died in 10 days in southern Afghanistan.
Sir Jock Stirrup said the Taliban were losing in Afghanistan
UK troops are involved in Operation Panchai Palang, or Panther's Claw, a major British assault against the Taliban in Helmand ahead of next month's Afghan elections. They have been joined by about 4,000 US and 650 Afghan troops.
Britain's Chief of Defence Staff, Sir Jock Stirrup, said it was important to "remember why our people are fighting in Afghanistan and what they are achieving through their sacrifice and their courage".
"The mission in Afghanistan is about supporting the delivery of governance in order to reduce the opportunities for extremist terrorist groups who are a direct threat to the United Kingdom, its citizens and their interests," he said.
"It's very clear to everyone who has visited Helmand in particular over the last three years that where we provide the necessary degree of security for its citizens real governance is starting to emerge."
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