A rocket attack forced the UN chief's plane to divert from Kabul on Tuesday despite a security lockdown imposed by thousands of troops to guard against attack on a major conference, officials said.
Draconian security measures paralysed much of the Afghan capital but saw the international meeting pass off without the kind of attacks that could have deeply embarrassed the Afghan government with the eyes of the world watching.
A nine-year Taliban insurgency is now at its deadliest in Afghanistan and the militia have carried out a string of deadly suicide and bomb attacks in the heavily fortified Afghan capital.
The toll rose on Tuesday after an Afghan soldier thought to be an army trainer shot dead two US civilians and a fellow Afghan soldier in the north of the country. Elsewhere a NATO soldier was killed in a bomb attack in southern Afghanistan.
Officials said UN Secretary General Ban Ki-moon and the foreign ministers of Sweden and Denmark were delayed arriving after a rocket attack on the airport as they prepared to land.
Sweden's Carl Bildt said on his blog that a plane carrying him and Ban was diverted to the main US base at Bagram and then flown by helicopter to Kabul shortly before the meeting featuring around 40 ministers.
"But we will be there for the opening of the conference. Basically this rocket attack represents little except for a serious attack on our night's sleep," Bildt said.
The Afghan interior ministry confirmed a rocket attack near the airport late Monday but give no details.
Separately NATO troops said they killed several insurgents allegedly preparing to attack the conference.
They were killed during a raid on a house on the southern edge of the city overnight, NATO's International Security Assistance Force (ISAF) said, suspecting the Taliban of plotting an attack on the conference.
Two other militants were captured while "pursuing a Taliban facilitator believed to be in final stages of preparation for attacks against the Kabul conference," it said.
ISAF had already said that others had been captured elsewhere in the city last week also said to be plotting attacks on the landmark meeting.
Thousands of Afghan and NATO forces on Tuesday led watertight security measures, shutting roads and banning traffic within several kilometres of the foreign ministry where the conference was taking place.
The airport was shut to commercial traffic. Monday and Tuesday have been declared public holidays and access to even pedestrians seriously restricted by armed police and soldiers manning makeshift checkpoints and security barriers.
In Kandahar, a man detonated explosives laden in a wheelbarrow, slightly injuring a passer by, said deputy city police chief, Fazil Ahmad Sherzad.
"I think he detonated accidentally," he told AFP at the site of the blast. The blast left a big crater on the road and shattered windows of the nearby shops, an AFP reporter said.
Karzai used the landmark conference, billed as the largest international gathering in Kabul, to say that his government was working to assume full leadership by 2014 and demanded greater control of aid money.