A co-ordinated Taliban assault on checkpoints in southern Afghanistan has killed five police officers and 25 Taliban fighters, local officials have said.

Omar Zwak, the provincial governor's spokesman, said that there were 500 to 1,000 Taliban fighters involved in fighting in Helmand province, which raged for 24 hourss.

However, the International Security Assistance Force (ISAF) issued a written statement which said that it had not seen anywhere near that number of Taliban fighters.

"Our reporting shows there were about 10 groups of four to five fighters each doing drive-by shootings against five police checkpoints," the ISAF statement said. ISAF also denied that the checkpoints were overrun.

'Success without NATO

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 spring offensive.

Al Jazeera's Jennifer Glasse, reporting from Kabul, said that amid the conflicting reports it was agreed that it was the largest Taliban attack since the group launched its offensive in April.

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 told 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."

Bomb kills police

The fighting came as officials said that a powerful roadside bomb killed seven policemen in the Chishti Sharif district of Herat province in western Afghanistan.

The policemen died when their vehicle hit the explosive device planted in the road on Tuesday morning, police said.

The officers were from the Afghan Public Protection Force, a government-run force that provides security for international supply convoys, aid groups and foreign-funded reconstruction sites.

The blast ripped through the vehicle as the men were heading to Obe district in Herat, where India is rebuilding a major hydroelectric dam.

Sher Agha, the district police chief, said that the explosion was so strong that the police truck was obliterated in the blast 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.