More than 100 people have been killed in a Pakistani air strike on a drug treatment centre in Afghanistan's capital, forensic laboratory sources told the BBC.

Some of the bodies were injured beyond recognition, sources at the Kabul Forensic Medicine department said. Taliban officials have put casualty figures much higher. The UN called for a swift investigation.

Pakistan denied striking the facility deliberately, saying it had "precisely targeted military installations and terrorist support infrastructure".

The cross-border conflict re-erupted last month, with Pakistan saying Afghanistan was harbouring militants who attacked its territory, something Kabul denied.

The UN Assistance Mission in Afghanistan (Unama) condemned the strike, which it said had been carried out by Pakistani forces on the Omid Addiction Treatment Hospital.

"Unama reiterates its call for de-escalation and an immediate ceasefire, and for parties to comply with their obligations under international law to ensure the protection of civilians," a statement said.

Some 2,000 people were being treated at the rehab centre, according to officials at the facility, who believe there could be hundreds of casualties.

The Afghan health ministry's spokesman, Sharafat Zaman Amarkhail, told the BBC there were no military facilities near the rehabilitation centre.

Residents reported hearing loud explosions across Kabul at around 20:50 (16:20 GMT) on Monday, followed by the sound of aircraft and air defence systems.

Family members of those being treated at the centrewere gathered outside, desperately trying to find out information about their loved ones.

A Taliban government spokesman said the death toll had reached at least 400, but the BBC has not independently verified the numbers. A BBC reporter at the scene saw more than 30 bodies carried out on stretchers on Monday evening.

Pakistan's information ministry said the strikes in Kabul and the eastern province of Nangarhar had been "precise and carefully undertaken to ensure no collateral damage is inflicted".

It dismissed Afghanistan's claim as a "misreporting of facts... [that] seeks to stir sentiments, covering illegitimate support to cross-border terrorism".

Formerly a US military base, the area where the strike occurred was a notorious hangout for drug users. When the Taliban returned to power in August 2021, they turned the space into a rehabilitation centre, housing users rounded up from across the capital.

Rescuers continued to search for survivors into Tuesday. In the morning, the extent of the damage - flattened debris littered with blankets and shoes beside charred, blown-out windows - was visible.

The latest violence follows months of clashes, despite the two sides agreeing to a ceasefire in October.

At least 75 people were killed and 193 injured in Afghanistan as a result of continuing cross-border fighting between the countries between 26 February and 13 March, according to Unama.

China, which has tried to cool tensions, said its Foreign Minister, Wang Yi, had spoken to his Afghan and Pakistani counterparts over the phone in the past week.

Calling for a ceasefire "at the earliest opportunity", Beijing on Monday urged the two countries to "remain calm and exercise restraint [and to] engage face to face" as soon as possible.