Afghanistan has deployed hundreds of commandos backed by NATO air strikes in Lashkar Gah to drive Taliban insurgents from the southern city after the militants killed 14 people in a coordinated attack, officials said Tuesday.

The assault Monday marked the insurgents' latest attempt to seize the capital of Helmand province, underscoring unravelling security as they expand their foothold across the opium-rich province.

"More than 300 commandos... have been deployed to the city to prevent Taliban advancement," said Abdul Jabar Qahraman, government special envoy for security in Helmand.

Provincial spokesman Omar Zwak told AFP the commandos were sent from Kabul and neighbouring provinces to launch a "clearance operation" in Lashkar Gah after the attack, which killed 10 policemen and four others.

"Soon the security forces will clear the whole city from Taliban," Zwak said.

"US enablers are in the area and will support as needed. Yesterday there were two air strikes in Helmand, both of these were in support of ANDSF (Afghan National Defense and Security Forces) strategic operations," a NATO spokesman told AFP Tuesday.

The Taliban have waged an insurgency against the western-backed Kabul government since being toppled from power by a US-led invasion in 2001. They have intensified attacks across the country in recent months, pressuring Afghan forces on multiple fronts.

Around 30,000 people have been displaced in Helmand in recent weeks and most have fled to Lashkar Gah. But the city is practically besieged, with roads from neighbouring districts heavily mined by the insurgents.

The intervention in Helmand has fuelled the perception that foreign powers are increasingly being drawn back into the conflict as Afghan forces struggle to rein in the Taliban.

Monday's early morning assault also underscored the Taliban's sustained push into urban centres, coming a week after they briefly stormed into Kunduz in the north before being repelled by Afghan forces.

The attack began with a car bombing in an attempt to break through the security belt around Lashkar Gah and enter the city, according to officials and local residents.

Mohammad Radmanish, a defence ministry spokesman, said security forces had managed to repel the attack and push back the insurgents, but it was feared they would return.

"We have enough forces on the ground now. Afghan air force and NATO's air support is also helping our forces," he said.

- US support -

On Saturday General John Nicholson, the NATO military commander in Afghanistan, flew with the Afghan defence minister to Lashkar Gah to assure provincial elders that the city would not fall.

The Taliban effectively control or contest 10 of the 14 districts in Helmand, the deadliest province for British and US troops over the last decade and blighted by a huge opium harvest that helps fund the insurgency.

In August Washington deployed some 100 troops to Lashkar Gah, the first major US deployment to the city since foreign forces withdrew in 2014.

In recent months the militants have attempted to overrun other provincial capitals, from Kunduz and Baghlan in the north to Farah in the west, but Afghan forces have managed to repel the attacks.

On Tuesday Taliban fighters renewed their offensive against Farah, triggering heavy fighting that lasted for hours.

A regional police spokesman Abdul Ahad Walizada said security forces drove back the attack, killing over a dozen insurgents, though he admitted sporadic fighting around the city continued into the night.

NATO officially ended its combat mission in December 2014, but US forces were granted greater powers in June to strike at the insurgents as President Barack Obama vowed a more aggressive campaign.