Kabul: At least 12 people including three US civilian contractors were killed Saturday when a suicide car bomber struck a NATO convoy, officials said, underlining the precarious security situation in the Afghan capital.

The Taliban denied responsibility for the blast, which struck outside a civilian hospital in Kabul following a wave of fatal bombings earlier this month that rattled the city.

The piercing explosion in a crowded residential neighbourhood reverberated around Kabul and left a trail of devastation, including twisted wreckage of burning vehicles with officials seen piling up bloodied bodies in a police pickup truck.

The blast killed 12 people and wounded 66 others, health ministry spokesman Wahidullah Mayar said on Twitter.

Senior health official Sayed Kabir Amiri confirmed the toll from the attack, which comes as Taliban insurgents escalate their annual summer offensive against the US-backed Afghan government amid faltering peace talks.

‘One Resolute Support (NATO) US contractor was killed and two Resolute Support US contractors died of wounds as a result of an... attack on their convoy in Kabul,’ NATO said in a statement.

‘Instead of seizing an opportunity to embrace peace, insurgents have again chosen violence in an attempt to remain relevant,’ a separate NATO statement said.

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

Taliban spokesman Zabiullah Mujahid said the group was not behind the attack, which prompted the heavily-fortified US embassy, located a few kilometres (miles) away in the centre of Kabul, to sound its emergency sirens and a ‘duck and cover’ alarm warning.

The insurgents are known to distance themselves from attacks that result in a large number of civilian casualties.

‘The mujahideen had no plan for an attack in Kabul today,’ Mujahid said.

‘Why are they killing us?’

Saturday’s blast comes amid heightened security in Kabul after a wave of bombings earlier this month that killed more than 50 people and wounded hundreds, prompting fury from President Ashraf Ghani who blamed Pakistan for failing to rein in Taliban insurgents.

The surge in lethal attacks has left the war-scarred city on edge.

Tempers flared at the scene of Saturday’s bombing, with a young Afghan man fighting back tears as he screamed: ‘Why are they killing us?’

The Taliban are stepping up their summer offensive, launched in late April, amid a bitter leadership dispute following the announcement of the death of longtime leader Mullah Omar.

Mullah Akhtar Mansour, Omar’s longtime trusted deputy, was named as the new Taliban chief in late July in an acrimonious power transition.

Al-Qaeda chief Ayman al-Zawahiri recently pledged his group’s allegiance to Mansour, in a move which could bolster his accession amid the growing infighting within the Afghan militant movement.

The latest wave of deadly violence underscores Afghanistan’s volatile security situation as peace talks appear to have stalled.

The first face-to-face talks between the Afghan government and the Taliban took place last month in the Pakistani hill town of Murree, aimed at ending the 14-year insurgency.

The Taliban distanced themselves from a second round of talks that were scheduled for the end of July after the announcement of Omar’s death.