At least 65 Afghan soldiers have defected to the Taliban, taking their weapons and equipment with them, and 88 have been killed in days of heavy fighting in the volatile southern province of Helmand, said the local provincial governor on Saturday.

The latest losses underline the heavy toll being exacted on Afghan security forces, now fighting largely alone since international troops ended most combat operations last year.

The police and soldiers have been engaged in near-continuous combat with insurgents for the past three weeks in the districts of Lashkar Gah, Marjah and Nadali in Helmand, one of the Islamist movement's traditional strongholds.

Although they have so far repelled Taliban efforts to take the provincial capital Lashkar Gah, they have not been able to push back the insurgents decisively from areas around the city.

"Soldiers from an Afghan army brigade in the Station area have joined the Taliban with their equipment and weapons," said Helmand Governor Mirza Khan Rahimi. He said a team had been sent to the town of Sangin to investigate the incident.

Taliban spokesman Qari Yousuf Ahmadi said in a statement five commanders and 65 army soldiers "repented for their mistakes and surrendered to the Mujahideen", bringing five armoured personnel carriers as well as weapons and ammunition.

Since the fall of the key northern city of Kunduz, which the Taliban briefly took over in September, the government of President Ashraf Ghani has come under mounting pressure over the worsening security situation.

Thousands demonstrated in Kabul this week demanding action after seven members of the Hazara ethic minority were brutally executed by Islamist militants.

On Saturday, more than 1,000 people demonstrated in the western city of Herat, said officials.

Afghan officials say the sharp reduction in surveillance and close-air support to troops on the ground has weighed particularly heavily since US forces cut their operational support to their Afghan allies.

President Ashraf Ghani repeated the need to strengthen Afghan air defences when he spoke by video conference with US Secretary of Defence Ash Carter on Friday.