Afghan President Hamid Karzai has announced that parliamentary elections will be delayed as Condoleezza Rice made her first visit to the country.
Afghanistan held its presidential election last year
President Karzai said the elections scheduled for May will take place in September due to "technical matters".
Ms Rice, the US secretary of state, said the poll would show "the Afghan people's commitment to democracy".
A bomb exploded in the southern city of Kandahar during her visit, killing five people and injuring another 32.
It was the worst bombing in the country since August of last year, Reuters news agency reported.
A police commander in Kandahar blamed the attack on the ousted Taleban movement, but a spokesman for the movement denied responsibility in comments to the Afghan Islamic Press.
The BBC's Andrew North in Kabul says one reason for the election delay is security concerns, and the Kandahar blast is a reminder of that risk.
Ms Rice held a joint press conference with Mr Karzai, the leader Washington chose to run the country's transitional government after the US helped Afghan forces oust the Taleban in 2001.
He easily won a presidential election last year.
Rice is on her first visit to Afghanistan
She vowed that the US would stand by Afghanistan, saying it had failed to do so after the Soviet withdrawal in 1989.
"We learned the hard way what it meant to not have a long-term commitment after the Soviet Union left. I think it is well understood that we did not remain committed," she said.
She said the September 11 2001 attacks on the US were "a joint tragedy of the Afghan and American people" resulting from the chaos that followed the Soviet withdrawal.
She also praised the Afghan people for their progress in coming out of decades of turmoil and civil war.
She held talks on reconstruction, drugs and the war against terrorism during her first visit to the country.
Ms Rice arrived in a C-130 Hercules military transport plane from Islamabad, and returned there later for further talks with Pakistani leaders.
In Kabul, Ms Rice also met Afghan Foreign Minister Abdullah Abdullah and leading US diplomats and military officials.
She discussed Afghanistan's growing trade in poppies, the raw material used to make opium.
The country produces almost 90% of the world's opium, according to the United Nations.
The US state department says Afghanistan is "on the verge of becoming a narcotics state" and "an enormous threat to world stability".
President Karzai has promised a "holy war" against the opium trade, which he describes as a "disgrace" to his nation.
Breaking the story
Ms Rice accidentally revealed the poll delay when she referred to "parliamentary elections that will take place this fall", apparently not aware the new date had yet to be made official.
When asked for clarification, Ms Rice said: "I hope I didn't break the story."
Mr Karzai then confirmed the elections would take place in September.
He said he had been informed of the decision by the head of Afghanistan's election commission.
"The preparations are going on and now they told us, the commission chairman, that the elections will be held in September," he said, according to the Associated Press news agency.
The delay, also blamed on the lack of an accurate census, had been rumoured for months.
The much-postponed vote was originally scheduled for June 2004, alongside Afghanistan's first presidential election which eventually took place in October.
Ms Rice's trip to South and East Asia will also take her to Japan, South Korea and China.
She has already visited India and Pakistan during her eight-day tour.