Armed men have killed at least 16 policemen in Afghanistan, beheading eight of them, according to officials, as security forces prepare to guard a second-round election.

Villagers in the southern province of Zabul found the decapitated bodies of eight local policemen who were seized two weeks ago, deputy provincial governor Mohammad Jan Rassoulyar told the AFP news agency on Wednesday.

The policemen were snatched by fighters after an attack on their convoy.

"Their bodies were discovered in Nawbahar district and we have sent elders to bring them to Qalat [the provincial capital]," Rassoulyar said, adding they were beheaded on Tuesday and found soon afterwards.

Taliban fighters, who are responsible for many attacks on Afghan police, were not immediately available to comment.

In the country's northeast, officials said that an attack on several police checkpoints in Badakhshan province had left eight officers dead.

General Fazeluddin Ayar, the police chief in the province, said on Wednesday that a large group of Taliban fighters had launched the attack in Yamgan district.

He told the AP news agency the fighting started late on Tuesday and was still going on. The police have sent reinforcement to the site.

Ayar says that along with the eight officers, at least five fighters had also been killed in the gun battle.

The Taliban have claimed responsibility for the attack. In a statement, they said that they raised their white flag above the Yamgan district headquarters.

Presidential election

The Taliban have pledged to target Afghan and foreign forces, as well as political efforts to end the conflict, as Afghanistan prepares to return to campaign mode on Thursday ahead of a second round run-off in the presidential election.

The election is a landmark after 13 years of violence that has killed at least 16,000 Afghan civilians and thousands more soldiers.

Nearly 3,500 members of the US-led coalition force have died since deployment in the country over a decade ago.

Providing security for the election will be a key test for about 350,000 Afghan police and soldiers who have taken over full responsibility from NATO combat troops.

Afghan forces were praised for preventing major attacks in the first round of the election.