The Afghan army has launched an offensive in the southern Taliban heartland to try to weaken the insurgents before the start of the new "fighting season", officials said.

Police and troops, in their first major assault since US-led NATO forces ended their combat mission last December, began targeting militants on Monday in four districts of restive Helmand province.

Helmand, a hotbed of the Taliban insurgency and a hub for drug trafficking, was the scene of repeated militant assaults during the fighting season last year.

The defence ministry said 76 insurgents were killed on the first day of the operation, which would also target militants in six districts in neighbouring Kandahar, Farah, and Uruzgan provinces.

"This is a totally Afghan-planned and Afghan-led operation. It will continue untill success is achieved," said General Abdul Khaliq, who commands the operation.

The Taliban's spokesman was not immediately available for comment.

The defence ministry said the assault, code-named "Zolfiqar", was the first in a planned series of large-scale operations by Afghan forces after NATO ended its combat mission in December.

"This is not the first and will be not be the last operation against the Taliban and their allies," said ministry spokesman General Dawlat Waziri.

Experts and Afghan military officials anticipate a surge in Taliban violence with the start of the traditional spring and summer "fighting season" in April or May.

The US and its allies have reduced their military presence in Afghanistan to a contingent of about 12,500 who are mainly focused on training and support for the 350,000 Afghan troops and police.