KABUL - At least 12 people including three Nato contractors were killed Saturday when a suicide car bomber struck a foreign forces 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 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.

A foreigner was among 12 people killed in the blast, with 66 others — including women and children — wounded, health ministry spokesman Wahidullah Mayar said on Twitter. He did not give the nationality of the foreigner. Senior health official. Security sources said the contractors worked for DynCorp International. The company, which provides training, security and aviation maintenance to the Nato mission and the Afghan military, did not immediately respond to a request for comment. Sayed Kabir Amiri confirmed that toll from the attack, which comes as Taliban insurgents escalate their annual summer offensive against the US-backed Afghan government.

“One Resolute Support (Nato) contracted civilian was killed in the attack and two others died of wounds as a result of the attack,” Nato said in a statement. A Nato spokeswoman told AFP that the contractors were not Afghan nationals, but did not specify their nationalities.

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. Saturday’s bomb was placed in a Toyota sedan, a security official at the scene said. Flames billowed from the car and parts of it were ripped apart by the blast and scattered along the street. Glass was blown out of the windows of the Shinozada hospital and a six-storey building opposite. On its website, the Shinozada is described as Afghanistan’s first private hospital.

The 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. Mulla 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 amid a faltering peace process.

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. The Taliban are fighting to overthrow the foreign-backed government, expel foreign forces from Afghanistan and impose a strict interpretation of Islamic law.

The violence has put paid to hopes that new leader Mansour would quickly return the insurgents to the negotiating table. Instead he seems set on consolidating his position in the group that ruled for six years until the 2001 US invasion.

Elsewhere in Afghanistan, the group has regained ground in parts of Helmand province since British and US combat forces it battled there left last year. It has also advanced in districts in the north but has struggled to hold ground.