At least 20 Afghan policemen have been killed after dozens of Taliban fighters stormed their outpost in the southern Helmand province.

The assault comes as the Taliban intensifies its summer offensive despite repeated government attempts to reopen peace negotiations.

Al Jazeera's Nicole Johnston, reporting from the capital Kabul, said another 16 policemen were wounded in Saturday's pre-dawn attack.

"We are told the attack was executed by hundreds of the Taliban fighters," she said.

"Over the last six weeks, most of the reports that we've heard about the Taliban attacks have actually been in the north and less in the south."

Nabi Jan Mullahkhil, Helmand police chief, said: "Dozens of armed Taliban attacked a police checkpoint in Musa Qala district of Helmand."

The raid reportedly occurred just after midnight and lasted several hours.

Taliban claim

The Taliban claimed responsibility for the attack, saying they seized several weapons, ammunition and other military hardware from the checkpost.

"Our mujahedeen, armed with heavy and light weapons, attacked police checkpoint in Musa Qala district," Qari Yousuf Ahmadi, Taliban spokesman, told AFP.

"In the attack, 25 police forces were killed and 13 others were wounded."

The Taliban, toppled from power in a 2001 US-led invasion, launched a countrywide offensive in late April, stepping up attacks on government and foreign targets.

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Saturday's attack marks another setback for Afghan forces, facing their first fighting season without full NATO support.

NATO's combat mission formally ended in December but a small follow-up foreign force of about 12,500 mainly US troops has stayed on to train and support local security personnel.

Afghan authorities have repeatedly tried to jumpstart talks with the Taliban in the hope of ending the 13-year conflict, but the fighters have set tough conditions, including the withdrawal of all foreign troops in Afghanistan.

Afghan security forces, many of them poorly equipped, have increasingly borne the brunt of the fighting around Afghanistan.

Badakshan attack

In early May, Taliban fighters killed at least 13 policemen after storming security outposts in the mountainous northeastern province of Badakhshan.

Zabiullah Mujahid, a Taliban spokesman, said the fighters seized control of several checkpoints and the centre of Yamgan district, killing several Afghan security personnel and seizing weapons.

Army forces tried to retake Yamgan last Sunday and managed to win back the district headquarters there.

The Badakhshan attack came just weeks after a similar Taliban raid on army checkpoints in the same province in which 18 soldiers were killed - including some who were beheaded.

Earlier this year a US watchdog said in a report that Afghan security forces were suffering heavy casualties on the battlefield and large numbers of troops were resigning or deserting their units.

Between October 2013 and September 2014, more than 1,300 Afghan army soldiers were killed in action and 6,200 were wounded, the Special Inspector General for Afghanistan Reconstruction (SIGAR) said in its report.

Between September 2013 and September 2014, more than 40,000 personnel were dropped from Afghan National Army rolls, it said.

The surge in attacks has also taken a heavy toll on civilians, according to the UN mission in Afghanistan.

Almost 1,000 civilians were killed in the conflict during the first four months of this year, a sharp jump from the same period last year, it said.