Prime Minister Gordon Brown warned on Friday that troops in Afghanistan faced a "very hard summer" after nine British soldiers were killed in as many days.
Speaking at the G8 summit in Italy after two more British soldiers were killed in separate incidents on Thursday, Brown said it was vital the international community saw through its commitment to Afghanistan.
Britain's death toll has increased sharply since its forces launched a major airborne assault, codenamed Operation Panther's Claw, last month against a Taliban stronghold near Gereshk in southern Helmand province.
Attacks by insurgents have also been increasing ahead of elections in Afghanistan next month.
In the latest incidents to hit the 8,300 British troops in the country, a soldier from the 4th Battalion The Rifles was killed in a "contact explosion" while on a foot patrol near Nad Ali in Helmand, the defence ministry said.
The other soldier, from the Princess of Wales's Royal Regiment attached to 1st Battalion Welsh Guards, died in a gun battle with insurgents near Lashkar Gah in Helmand.
A spokesman for Task Force Helmand, Lieutenant Colonel Nick Richardson, said: "These fine British soldiers paid the ultimate sacrifice and their memory will live with us forever.... We know that their deaths were not in vain."
The fatalities bring to 178 the total number of British service personnel who have died during operations in Afghanistan since the US-led invasion in 2001.
From the G8 summit venue of L'Aquila in central Italy, Brown sent his condolences to the families of the dead soldiers, but said Britain and its international allies in the NATO-led ISAF force would stick to their task.
"This is a very hard summer, it's not over but it's vital that the international community sees through its commitment," Brown said.
"There's a recognition that this is a task that the world has got to accept together."
Britain's "resolution to complete the work that we have started in Afghanistan and Pakistan is undiminished," Brown added.
The human cost of that task was underlined when the bodies of five soldiers were flown home to Britain on Friday. Four were killed while taking part in Operation Panther's Claw.
A lone bugler played "The Last Post" as the men's coffins, each draped in a Union Flag, were taken off the plane at RAF Lyneham in Wiltshire, southwest England.
Hundreds of mourners lined the streets as hearses carrying the coffins travelled through the streets of nearby Wootton Bassett.
In an unusual breach of the cross-party consensus on Afghanistan, the leader of the opposition Liberal Democrats, Nick Clegg, warned this week that soldiers' lives were being "thrown away" in Afghanistan.
Clegg said politicians were setting the troops impossible goals.
Meanwhile, the mother of a soldier killed in Iraq won the right Friday to a judicial review of the British government's refusal to hold a public inquiry into the use of Snatch Land Rovers there and in Afghanistan.
Amid allegations that the vehicles are too lightly armoured to resist roadside bombs, London's High Court heard 37 soldiers had died in Iraq and Afghanistan while using the vehicles since 2005.
Susan Smith, whose son Phillip Hewett died in 2005 at the age of 21, is challenging the use of the vehicles.