International donors have said they will provide nearly $2bn (£1.1bn) in aid to help transform Afghanistan.
The UK said it remained committed to delivering stability to Afghanistan
The US has promised $1.1bn while the UK says it will give $800m, as part of a five-year development plan being discussed at a conference in London.
Afghan President Hamid Karzai said his country had made great strides towards peace and democracy, but warned drugs and terrorism remained grave threats.
Officials from about 70 nations are attending the two-day gathering.
The plan, known as the "Afghan Compact", aims to promote stability and development in Afghanistan in return for economic and military support from the international community.
The BBC's Nick Childs in London says one major theme at the conference has been that the world as a whole still has a major direct stake in Afghanistan's future.
With more than four years passed since the US-led overthrow of former Taleban regime, he says a key question is whether the outside world will back up its pledges of support with real commitments.
Speaking at a news conference on the first day of the meeting, UK Foreign Secretary Jack Straw announced Britain's pledge.
He said the money would be made available over the next three years.
Mr Straw said the conference had set ambitious but achievable goals.
"No-one underestimates the many problems the country still faces", he said, "and they require long-term investment and commitment".
"The international community," he said, "remains strongly committed - indeed more strongly than ever - to supporting the people of Afghanistan for the long-term".
The financial pledge followed an earlier announcement by US Secretary of State Condoleezza Rice that the Bush administration planned to give Afghanistan $1.1bn in aid next year.
"The transformation of Afghanistan is remarkable but incomplete," she said. "And it is essential that we all increase our support for the Afghan people."
Russian Deputy Finance Minister Sergei Storchak said Moscow would write off Afghanistan's $10bn Soviet-era debt.
Mr Karzai said Afghanistan needed $4bn in international aid annually.
'Road to success'
Our correspondent says Afghanistan's huge illegal drugs trade remains a major concern among the international community.
Afghanistan is the source of nearly 90% of the world's opium and heroin, with the illegal drugs trade accounting for a third of the country's economy.
President Karzai said it would take at least 10 years to eradicate poppy growing in his country as part of the counter-narcotics effort.
But Mr Karzai said: "Afghanistan would like to continue on its road to more success, prosperity and Afghanistan will remain a great asset for security in the region and the globe."
Earlier, UK Prime Minister Tony Blair said the international community was determined to stick with the Afghan people through a "struggle that concerns all of us".
UN Secretary General Kofi Annan echoed that theme, but also warned that Afghanistan's future remained in the balance.
"Afghanistan today remains an insecure environment," he said.
"Terrorism, extreme violence, the illicit narcotics industry and the corruption it nurtures threaten not only continued state-building, but also the fruits of the [December 2001] Bonn process."
The conference comes as Nato prepares to expand its role in Afghanistan with the deployment of an extra 6,000 troops amid renewed concern about the level of violence in the country.
The troops will move into the volatile south of the country where US-led troops have been operating.