His comments came after three soldiers were killed in a suicide attack.
The men, from 2nd Battalion the Parachute Regiment, were on foot patrol in Helmand when the blast happened.
Defence Secretary Des Browne added no loss could be compensated for, but he remained convinced the campaign was "the noble cause of the 21st century".
"We are making significant progress in Afghanistan. It's slow, sometimes it's frustratingly slow," he told BBC Radio Four's Today programme.
"The reason we are making that progress is because those very brave and professional young men and women have gone about their job in such a way that they have created, in the most difficult of circumstances, a degree of security that many people a couple of years ago thought was impossible."
The prime minister said those who had died had "paid the ultimate price" but had "achieved something of lasting value."
He said: "My first thoughts and condolences are with the families of these soldiers, who died serving in Afghanistan with such distinction more:...
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