The Prime Minister insisted today that British forces had the resources needed to "do the job" in Afghanistan.
After a rising toll of deaths and injuries in recent days, Mr Brown acknowledged it had been a "sad and difficult time" for our armed forces and the country.
But he said that if the Taliban's "vicious insurgency" was to be defeated, British troops must "persist" with their mission.
Mr Brown said: "It has been a very difficult summer and it is not over yet.
"But if we are to deny Helmand to the Taliban in the long term; if we are to defeat this vicious insurgency and by doing so make Britain and the world a safer place, then we must persist with our operations in Afghanistan.
"I am confident we are right to be in Afghanistan - that we have the strongest possible plan and we have the resources needed to do the job."
In a sombre statement to MPs on the G8 Summit, Mr Brown sent condolences to the families of those who had died in fighting over recent days, saying the country owed them a "huge debt of gratitude".
He said: "This has been a sad and difficult time for our armed forces and for our country."
After naming those who had lost their lives, he said: "It's at times of loss and sadness like these that we become ever more aware of the service and sacrifice our armed forces make for our country.
"This is a time of great challenge for our armed forces serving in Afghanistan."
Time was to be made available for a Commons debate on Afghanistan on Thursday, he disclosed.
Eight years ago, after September 11 2001, the case for intervention in Afghanistan was clear - to remove the Taliban regime and deprive al Qaida of a safe base for terrorism.
"In 2009, the case for our continued involvement is the same: to prevent terrorist attacks here in Britain and across the world, by dealing with the terrorist threat at its source - that crucible of terror on the border and mountain areas of Pakistan and Afghanistan."
Mr Brown said three quarters of terror plots against the UK had roots in the border between Afghanistan and Pakistan.
"To succeed we must succeed both in Afghanistan and Pakistan."
In Afghanistan international forces must take the lead because the Afghan army and police were not yet able to maintain control alone.
Currently, about 9,100 British troops were involved in the military operation.
"We keep our force levels under constant review. And I have been reassured by commanders on the ground and at the top of the armed services that we have the manpower we need for the current operations."
Mr Brown said he had yesterday spoken to Afghanistan President Hamid Karzai and urged him to make more Afghan army personnel available for operations in Helmand, "so that our hard-won gains can be fully consolidated".
British troops would continue to face a "tough and dangerous battle and we will continue to give their safety the highest priority".
Funding for operations had risen to over £3 billion this year and Chancellor Alistair Darling had made clear "all urgent operational requirements will be met".
Helicopter numbers had increased by 60 per cent in the last two years and the Government planned to go further this year with the deployment of new Ridgeback vehicles and Merlin helicopters.
"Despite the tragic losses of the last two weeks, our commanders assure me that we are having a major impact on the Taliban in central Helmand and that morale is high."
All the G8 members agreed on the importance of the work being done in Afghanistan. "I talked directly with President Obama about the challenges we face together."Reuse content