Armed men attacked a convoy of fuel tankers in Dushi district of Baghlan province [EPA]
A roadside bomb has killed five border policemen in southern Afghanistan amid a surge in violence in the country.
Officials said on Friday that the policemen were hit when a mine blew up their vehicle in Spin Boldak district in Kandahar province, late on Thursday.
The AFP news agency said the Taliban had claimed responsibility via a text message for the Spin Boldak blast.
Also on Thursday, one NATO soldier was killed in the east and a nephew of Gulbuddin Hekmatyar, the founder of Hizb-i-Islami, died in Wardak province.
Habibullah Shoab Hekmatyar, a 17-year-old former student in Peshawar, Pakistan, had joined the fight against international forces in Wardak.
Harun Zarghun, a spokesman for Hizb-i-Islami, said the nephew was killed in a coalition airstrike along with one of his colleagues in Nirkh district, Zarghun said.
NATO confirmed that there was a coalition airstrike Thursday in Nirkh, but could not confirm if Hekmatyar's nephew was among four who were killed.
The attack in Spin Boldak is the latest in a wave of attacks against Afghanistan's police and army in the run-up to the start of the transition from foreign to Afghan security control in July.
On the Pakistani side of the border, two Pakistani policemen were reportedly killed when hundreds of armed fighters came across from Afghanistan's Kunar province.
In Khost, another province that borders Pakistan, three Afghan men who were part of a civilian police force guarding a road construction project were killed on Thursday in Spera district, the AP news agency reported an official as saying.
The men, who were part of the so-called Afghan Public Protection Force, opened fire as a NATO helicopter passed overhead.
Abdul Hakim Ishaqzai, a Khost police chief, said the men were killed by Nato forces, but the alliance has not yet commented on the incident.
Armed men killed the driver of a NATO supply truck on Friday after opening fire on a convoy heading to Afghanistan from Pakistan's troubled southwest, officials said.
An unknown number of attackers on motorcycles signalled for the convoy to stop and opened fire after drivers ignored them, before the gunmen fled the scene, local official Mohammad Azam told the AFP news agency.
The incident took place in Baghbana town of Khuzdar district, 250km south of Quetta, the main city of Baluchistan province, Azam said.
"A driver of one NATO supply truck was killed, all other drivers and their helpers remain safe," he said by telephone.
NATO supply trucks and oil tankers are targets of frequent attacks blamed on fighters attempting to disrupt supplies for more than 130,000 international troops fighting in Afghanistan.
In the north of Afghanistan, armed men attacked a convoy of fuel tankers on Thursday night in Dushi district of Baghlan province.
The governor's office said two tankers, carrying fuel for international forces, were burned and seven others were damaged.
The coalition said the convoy was operated by a civilian contractor.
When the fighters attacked, the convoy's security guards returned fire, according to NATO.
The coalition said no casualties were reported, but three vehicles were destroyed.
Addressing the Pentagon on Thursday, Robert Gates, the US defence secretary, said US troops could reach a turning point in Afghanistan if they can sustain hard-won security gains.
"It's possible that, by the end of this year, we will have turned a corner, just because of the Taliban being driven out and, more importantly, kept out," he said.
Calling 2011 a "critical year", he said military commanders were expecting an increase in Taliban activity in May and June after the poppy harvest ends, but that troops could turn a corner if they were able to hold positions they had won back from the Taliban.