In a statement, the US state department said the aid would be used to build schools, roads, and hospitals, help farmers improve their ability to raise crops and deliver them to the marketplace, stimulate new energy infrastructure and strengthen democracy.
A full tally of pledges is expected to be announced at the end of the meeting.
"Without stability in Pakistan, there is no stability in Afghanistan either"
Taro Aso, Japan PM
The US move comes a day after Japan pledged up to $1bn in aid to Pakistan, also over two years, for economic reforms and its fight against terrorism.
Taro Aso, the Japanese prime minister, said the world was still facing the threat of terrorism years after the September 11 attacks on the United States.
"Even now, tragedies are being repeated in Islamabad, Lahore, Mumbai and Kabul," he said in a speech opening the one-day donors' conference.
"Terrorism is posing a threat to the international community, and we cannot help but acknowledge that efforts to eradicate terrorism are now at a crucial stage."
Aso said the international community supported the strategies taken by Pakistan and Afghanistan to ensure stability along the shared border.
"Without stability in Pakistan, there is no stability in Afghanistan either."
He also acknowledged Pakistan's "vitally important role" in international efforts to counter terrorism and extremism.
'Ready to fight'
|Asif Ali Zardari said the extent of danger facing Pakistan is still not widely understood [AFP]
Asif Ali Zardari, the Pakistani president, in his address at the opening of the meeting stressed his resolve to fight "terror".
"In spite of the fact that I lost the mother of my children, I have taken up this challenge," Zardari, whose wife, former prime minister Benazir Bhutto, was killed in a December 2007 attack, said.
"There is a desire to help Pakistan," the Pakistani leader said. "But I feared, I still fear, that the understanding of the danger that Pakistan faces still does not register fully in the minds of the world."
"It does not end on my border. If we lose, you lose. If we are losers, the world is a loser."
Economic improvement in Pakistan is seen as a key not only to preventing the spread of poverty, but also to slowing the growth of terrorism, which depends on the poor to fill its ranks.
Economists say 40 per cent of Pakistan's 160 million people live on $1 a day or less. The government puts the figure at 33 per cent.
Hoda Abdel Hamid, Al Jazeera's correspondent reporting from Islamabad, said: "The idea behind this conference, is that if you want to stabilise Afghanistan, you have to stabilise Pakistan.
"That is why all these countries - at a time when there is a global economic crisis - are putting a fair amount of money into Pakistan, to invest in infrastructure projects, to create jobs and to get the population feeling that they are part of Pakistan again."