KANDAHAR, Afghanistan - Two policemen and two security guards have been killed in separate incidents in Afghanistan in the latest in an upsurge in Taliban-linked attacks, officials said Thursday.

A bomb fixed to a motorbike exploded Thursday outside the gates of a private construction company in southern Kandahar city, said Ghulam Mohammad, a police officer at the scene of the blast.

"Two guards have been killed and three other civilian workers of the company have been wounded," Mohammad said.

The blast hit close to the Kandahar provincial police headquarters. In eastern Khost province overnight, Taliban insurgents attacked an Afghan border police convoy, sparking a clash.

"Two border policemen were killed in the Taliban attack," Shair Ahmad Kochai, the provincial border police chief, told AFP. Police arrested 15 suspected militants in the aftermath of the attack, he added.

Police blamed both attacks on "enemies of the people of Afghanistan" - a reference to the Taliban, who were ousted from power in late 2001 by a US-led offensive but are now waging a fierce insurgency.

Meanwhile in the normally peaceful northern Kunduz province, a roadside bomb struck a vehicle of aid organisation Mercy Corps, wounding three people, said Mohammad Nasir Foshanji, who is in charge of security for the group. "Their vehicle was struck by a roadside bomb in which three of them including the driver were slightly wounded," Foshanji said.

The Taliban are the main militant group operating in Afghanistan. With support from Al-Qaeda, they are fighting the US-backed government of President Hamid Karzai in an insurgency that has seen a record number of attacks this year.