Taliban insurgents killed nine people in multiple attacks on police targets in Afghanistan Thursday, in their first major assaults since an acrimonious power transition after the announcement of leader Mullah Omar's death.

In the first attack a suicide truck bomber killed six people early Thursday when he struck a police compound in Pul-i-Alam, the capital of insurgency-racked Logar province just south of Kabul.

The force of the explosion damaged government buildings near the site, which was left littered with debris and shards of broken glass.

The bombing highlights growing insecurity amid a faltering peace process with the Taliban as Afghan forces face their first summer fighting season without full NATO support.

"A water truck filled with explosives was detonated when it was stopped at the gate of the Quick Reaction Force (police) compound," said deputy provincial police chief Mohammad Qari Wara.

"It was a powerful explosion... which killed three members of the Quick Reaction Force and three civilians."

President Ashraf Ghani condemned the "cowardly act" in a statement, adding that the "killing of innocent people is an unforgivable crime that can be justified in no religion".

In two other separate attacks in the southern province of Kandahar, Taliban militants raided a police checkpoint and a police station, sparking brief gun battles that killed three local security officials, including an intelligence officer.

Meanwhile, at least 17 people, including 12 Afghan army soldiers, were killed in a helicopter crash in the volatile southern province of Zabul, although officials said the incident was caused by a technical fault and not insurgent activity.

The attacks in Logar and Kandahar mark the first major insurgent assaults since the Taliban confirmed last week the death of their leader Mullah Omar, who led the militant movement for some 20 years.

The Taliban are stepping up their summer offensive despite an increasingly bitter power transition within the militant movement after Mullah Akhtar Mansour was announced as the new leader on Friday.

The group claimed responsibility for all three attacks, with spokesman Zabiullah Mujahid alledging that more than 100 security personnel were killed in the Logar bombing.

The insurgents are known to exaggerate the death toll in attacks on Afghan government and military targets, but do not usually claim responsibility for those which result in a large number of civilian deaths.

Surging civilian casualties

Yet in a rare admission, Mujahid said "some civilians may have been wounded (in Logar) as a result of broken glass".

Civilian casualties in Afghanistan hit a record high in the first half of 2015, a UN report said Wednesday, as Afghan forces struggle to contain the expanding conflict six months after the NATO combat mission ended.

The report said 1,592 civilians were killed, a six percent fall from last year, but the number of injured jumped four percent to 3,329. Overall, casualties reached their highest level since the UN began issuing its authoritative reports in 2009.

The statistics are a grim indicator of the rising violence as the Taliban insurgency spreads north from its traditional southern and eastern strongholds, with Afghan forces increasingly battling the militants on their own.

US-led NATO forces ended their combat mission in Afghanistan in December, but a 13,000-strong residual force remains for training and counter-terrorism operations.

The Taliban face growing internal divisions after Mullah Mansour's appointment was announced.

Some top leaders, including Omar's son and brother, have refused to pledge allegiance to Mansour, saying the process to select him was rushed and even biased.

Tayeb Agha, the head of the Qatar political office set up in 2013 to facilitate talks with Kabul, resigned on Monday in protest at Mansour's appointment and on Thursday two more members followed suit.

The power struggle has cast a pall over a fragile peace process aimed at ending Afghanistan's long war.

The Taliban have distanced themselves from the second round of talks with the Afghan government that were scheduled for last Friday, but were cancelled after the announcement of Omar's death.