UK forces' mission in Afghanistan is showing "signs of success", despite the deaths of 15 soldiers in 10 days, Prime Minister Gordon Brown has insisted.
To stop fighting the Taliban would make the UK "less safe" because a "chain of terror" linked the nations, he told the British Forces Broadcasting Service.
The government denies claims that UK troops lack manpower to hold any ground gained as part of the latest offensive.
US President Barack Obama has described Britain's contribution as "critical".
The president paid tribute to the latest UK casualties, telling Sky News the increasingly deadly conflict was a "serious fight" that was essential for the future stability of Afghanistan.
See a graph of UK troop deaths in Afghanistan
Mr Brown gave his interview after the Ministry of Defence released the names of two British soldiers killed in Afghanistan on Thursday.
Rifleman Daniel Hume, 22, of 4th Battalion The Rifles, and Private John Brackpool, 27, Prince of Wales' Company, 1st Battalion Welsh Guards died during the recent offensive in Helmand - Operation Panther's Claw.
The prime minister praised the "professionalism, courage and sense of duty" of all Britain's armed forces.
US president sets Afghan target"I think the operation in Helmand province is showing signs of success," Mr Brown said.
"Our troops are making progress as they attempt to make the area safer. Despite the losses, our forces are doing a magnificent job in moving forward."
Justifying the continued UK military presence in Helmand, he said: "It comes back to terrorism on the streets of Britain.
"If we were to allow the Taliban to be back in power in Afghanistan, and al-Qaeda then to have the freedom of manoeuvre it had before 2001, then we would be less safe as a country.
"There's... a chain of terror, that links what's happening in Afghanistan and Pakistan to the streets of Britain.
"Three quarters of all terrorist plots that our security services have had to deal with have come from those mountainous areas."
'More troops'
Opposition leaders have accused the government of failing to properly equip troops.
The BBC's Martin Patience said troops were increasingly threatened by improvised explosive devices (IEDs) - or roadside bombs.
"What the soldiers and officers want is more helicopters. That gives them greater manoeuvrability around Helmand province," he said.
"It also get soldiers off the roads and away from these IEDs. They want more vehicles with better armour."
Mr Brown said 1,000 more vehicles had been sent since last year and by the end of 2009 new Ridgeback armoured vehicles and Merlin helicopters would also be added to the armoury.
But Tory former defence secretary Lord King said the Americans had eight times as many helicopters for the number of personnel.
"A really critical shortage is helicopters," he said, adding that the UK had "never had enough troops to do the job".
A former commander of British troops in Bosnia, Col Bob Stewart, also said more soldiers were needed in Afghanistan.
"Commanders there have repeatedly asked for more. The actual figure some of them have requested is 4,000," he said.
"It's all very well going out on these operations. It's absolutely futile if you take the ground at considerable cost... then actually just withdraw because you haven't got the manpower to hold the ground."
But armed forces minister Bill Rammell told the BBC troop numbers had been increased from 5,500 to 9,000 and that the international force had been bolstered by an extra 20,000 US personnel.
"We keep troop levels under review but we believe we have got the numbers to do the job," he said.
Meanwhile, the Stop the War coalition has announced a protest in London on Monday, calling for troops to be withdrawn from Afghanistan in light of escalating losses.
'Wrong battlefield'
Author James Fergusson, who has written about Britain's role in Afghanistan, told the BBC the UK was "fighting on the wrong battlefield".
"Al Qaeda were thrown out of Afghanistan in 2001/2. They are in Pakistan. There's never been a Taliban bomb on the streets of London," he said.
One hundred and eighty-four service personnel have died in Afghanistan since 2001, more than the 179 who were killed during the war in Iraq.
On Friday in Helmand, five soldiers from the 2nd Battalion The Rifles were killed in two separate blasts near Sangin, while a member of the 2nd Royal Tank Regiment died near Nad Ali.
BRITISH CASUALTIES IN AFGHANISTAN MARCH 2006 - JULY 2009
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