A powerful remote-controlled bomb destroyed a UN vehicle in southern Afghanistan's main city, killing four Nepalese guards and an Afghan driver, officials said.
The attack on a three-vehicle UN convoy in Kandahar on Tuesday was the bloodiest in Afghanistan for the world body since the hard-line Taliban militia's 2001 ouster, and illustrated how violence still impedes much-needed reconstruction.
The convoy was beside a canal when unidentified assailants detonated the charge. It hit a gray sport-utility vehicle, killing the four guards and their driver, police and the UN said.
A reporter saw two charred bodies lying on the road nearby. The explosion blew off two of the car's doors and gouged a crater in the road.
While there was no immediate claim of responsibility, the attack came a day after a Human Rights Watch report accused Taliban militants of committing war crimes by targeting civilians.
The rights group said militants killed nearly 700 Afghan civilians last year -- more than three times the civilian deaths attributed to US and NATO forces, which have been criticized for using excessive force in civilian areas.
Violence in the south and east has created a vicious cycle for President Hamid Karzai and his international backers: Militants and criminals scare off aid agencies, fueling resentment against the government, especially among ethnic Pashtuns, from whom the Taliban draws its main support.
UN spokesman Adrian Edwards said of Tuesday's attack that "the problem, of course, is that insecurity, threats and incidents like this make our job more difficult."
With fighting picking up after a winter lull, aid and development agencies "can become a collateral victim ... and that is a new concern," said Lex Kassenberg, country director for CARE International.
The Taliban's increasing use of suicide bombers has drawn comparisons with Iraq.
A video purportedly shows a senior Taliban leader, Mullah Dadullah, addressing about 200 young suicide bombers.
"Today, we know better that infidels fear martyrdom attacks more than anything else. They have nothing to resist it," Dadullah told the recruits.
He made no distinction between military and civilian targets in the video, which was undated and gave no indication of where it was made.
There have been 39 suicide attacks in Afghanistan this year, a threefold increase from a year earlier, according to Afghanistan's NGO Safety Office, which advises relief groups on security.
US forces recently intercepted Iranian-made weapons intended for Taliban fighters in Afghanistan, the Pentagon's top general said on Tuesday, suggesting wider Iranian war involvement in the region.
It appeared to be the first publicly disclosed instance of Iranian arms having entered Afghanistan, although it was not immediately clear whether the weapons came directly from Iran or were shipped through a third party.
Marine Corps General Peter Pace, chairman of the Joint Chiefs of Staff, said that unlike in Iraq, where US officials say they are certain that arms are being supplied to insurgents by Iran's secretive Quds Force, the Iranian link in Afghanistan is murky.
"It is not as clear in Afghanistan which Iranian entity is responsible, but we have intercepted weapons in Afghanistan headed for the Taliban that were made in Iran," Pace told a group of reporters over breakfast.