KANDAHAR: At least 20 Afghan policemen were killed on Tuesday in a Taliban attack near Kabul, bringing to 26 the number of officers to die since the start of an army offensive aimed at weakening the insurgents.

The assault saw Taliban gunmen and suicide attackers strike police headquarters in Puli Alam city, south of Kabul. Provincial police chief Abdul Hakim Ishaqzai told AFP that in addition to those killed, nine were wounded.

The attack came as 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 restive Helmand province, a hotbed of the Taliban insurgency and a hub for drug trafficking.

The defence ministry said 76 insurgents were killed on Monday, the first day of the operation, which will also target militants in six districts in neighbouring Kandahar, Farah, and Uruzgan provinces. The offensive is designed to hurt the Taliban before the start of the so-called “fighting season”.

“This is a totally Afghan-planned and Afghan-led operation. It will continue until 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. Some observers have warned that unusually early warm weather this year may have already encouraged militants to venture out of their hideouts in the rugged mountainous area along the border with Pakistan.

'2015 will be a trying year'

Highlighting the dangers to Afghan forces, at least 20 policemen were killed in Tuesday's attack in Puli Alam. Niaz Mohammad Amiri, acting governor of Logar province where the attack took place, confirmed the provincial police chief's toll.

Taliban militants also carried out an ambush at an abandoned building in the Maiwand province of southern Kandahar Monday night, killing six policemen after luring them to the site by setting off a bomb.

“They fired as (police) arrived at the house. Unfortunately six police were martyred,” provincial police spokesman Zia Durrani told AFP.

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.

Many have warned that the Nato withdrawal will embolden the Taliban to beef up their insurgency, still resilient more than 13 years after the US-led invasion ousted them from power in Kabul.

"We know it, the Americans know it, they have said that the Taliban will intensify their attacks this year, now that most Western military forces have left," retired Afghan army General Atiqullah Amarkhail said. "2015 will be a trying year for the Afghan armed forces as they bear the full brunt of Taliban efforts and they have to be highly prepared."

Violence across Afghanistan hit record levels in 2014, with civilian and military casualties the worst since the 2001 invasion.

According to United Nations data, more than 3,000 civilians were killed in 2014, up 18 per cent on 2013, while the Pentagon said nearly 5,000 Afghan soldiers and police were killed last year.

The country is also grappling with political instability after the Afghan parliament last month rejected most of President Ashraf Ghani's nominees for the new cabinet.