Condoleezza Rice was in Afghanistan for a short stopover for talks on the US-led "war on terror," including the fight against the Taliban, whose nearly five-year battle against the government is going through its deadliest phase yet.
The top US diplomat said at a news conference with Afghanistan's president, Hamid Karzai, that the international community stood behind his government's fight against a "common enemy" responsible for attacks around the world.
The Taliban and al-Qaeda, which was sheltered by the Taliban government toppled by a US-led coalition in 2001, may have changed tactics but Kabul's allies were working together to find new strategies against them, she said.
"Afghanistan has determined enemies. They are ruthless but they will not succeed," she said after talks with Karzai at the presidential palace. "They are simply not going to win ... we will not allow it to happen."
"This is a thinking enemy that is changing its tactics too," she added. "We are making great progress, we share our views about changes that might be made or tactics or even strategies."
Also on Wednesday, a car bomb blew up prematurely as it approached a US military convoy in Afghanistan, killing two people.
The attack in the southern province of Zabul came as Rice was in the capital, Kabul.
Provincial government spokesman Gulab Shah Alikhail said two people in the car were killed. No Americans were hurt, he said.
"The bomb in their car went off prior to hitting the convoy," he said.
A Taliban commander in Zabul, Mullah Mohammad Massoum, claimed responsibility in a telephone call from an undisclosed location.
Massoum said only one Taliban bomber carried out the attack and the other person killed might have been a passer-by. He said civilians should stay clear of military convoys.
Rice arrived from an overnight stay in Islamabad, where she called for greater co-operation between the often bickering neighbours.
"Pakistan is a friend of the United States and a fierce fighter in the war on terrorism. Afghanistan also is a friend of the United States and a fierce fighter in the war on terror," Rice said at a news conference late on Tuesday.
Pakistan and Afghanistan have been in disagreement for months over claims by Kabul - and sometimes the US-led coalition - that Islamabad has failed to crack down on fighters carrying out cross-border attacks from its soil.
Pakistan has deployed about 80,000 troops in tribal regions to hunt down Taliban and al-Qaeda fighters who sneaked in after the topple of the hardline government.
Rice has had nothing but praise for Karzai and Pakistan's leader, General Pervez Musharraf, who abandoned Pakistan's support for the Taliban after the September 11, 2001, attacks in the United States.
Her visit comes at a difficult time for Karzai, with Afghans increasingly frustrated about the insurgency and the lack of development since the Taliban were forced out.
Rice had praise for Pakistan's
leader, Pervez Musharraf
The capital was shaken by violence a month ago.
Critics say Karzai's power barely extends beyond Kabul, with much of Afghanistan in effect ruled by drug barons, regional commanders or the resurgent Taliban.
Rice last visit to the the region was three months ago, when she accompanied the US president, George Bush.