A powerful roadside bomb has killed seven policemen in western Afghanistan as forces were engaged in a deadly battle with large groups of Taliban fighters who attacked police checkpoints in the south, officials have said.

The policemen died when their vehicle hit the explosive device planted in the road in the Chishti Sharif district of Herat province on Tuesday morning, police said.

Sher Agha, the district police chief, said that the explosion was so strong that the police truck was obliterated in the blast in Obey district and there were no survivors.

There was no claim of responsibility for the deadly assault on the policemen, who were guards of the Salma dam project and were on their way to Herat city. However, similar bombs have often been blamed on the Taliban.

The blast came as a spokesman for the Helmand governor said the Taliban launched attacks in the Sangin district of Helmand province.

Al Jazeera's Jennifer Glasse, reporting from Kabul, said that the interior ministry said five police had been killed in battles which had raged for 24 hours there.

Conflicting reports

However, there were conflicting reports on how many Taliban fighters were involved in the latest assaults.

"The provincial governor's spokesman says that it's between 500 and 1,000 fighters," Glasse said.

"NATO's International Security Assistance Force here, ISAF, issued a statement saying its reporting puts those numbers far lower, the Taliban numbers at a hundred maximum and that they were drive-by shootings, so two very different stories."

Our correspondent said local officials on the ground told Al Jazeera the Taliban attacked police checkpoints about 5am on Monday and took over three checkpoints.

However, the officials said Afghan security forces had called in reinforcements and Afghan national police and the Afghan army had driven the Taliban back.

"What is agreed between the Taliban and the Afghan government is that this is the largest Taliban attack in their spring offensive that they launched last month," Glasse said.

'Success without NATO'

Glasse said it was significant that Afghan security forces did not ask for NATO assistance.

"This, of course, is the year that Afghan security forces take control of security of the entire country, NATO watching to see how they'll do," she said.

"The Afghan provincial government spokesman in Helmand province tells Al Jazeera that actually this is a success, they've pushed back the Taliban and it shows that the Afghan security forces can do well on their own."

A government spokesman said earlier that four Afghan soldiers and 26 Taliban had been killed in the battle overnight and five others were wounded in the deadly clashes.

Qari Yousuf Ahmadi, spokesman for the Taliban, told Al Jazeera that a group of local Taliban had captured three government checkpoints during the attacks.

He said the Taliban had killed a number of Afghan security forces, including two top Afghan police commanders in the area as part of its Khalid-bin-Waleed spring offensive.