Two suicide explosions hit a line of Afghan police buses in the western part of Kabul on Thursday, leaving at least 30 people dead, authorities said.
The attack, claimed by the Taliban, occurred at around midday in Qala-e-Haidar Khan, an area on the western outskirts of Kabul, and also injured nearly 60 others.
Earlier media reports said that at least 40 people were killed in the attack.
"Five green-coloured police buses were running along Kabul-Kandahar road. One suicide car bomber struck one bus at the middle of the convoy, forcing the vehicles to stop," witness Mohammed Karim told Xinhua.
"The second bomber arrived and detonated his car bomb roughly 20 minutes later after arrival of rescue teams and people."
Further details about the incident were still forthcoming amid the absence of any official statement.
Sediq Sediqqi, spokesman of the Interior Ministry, told local media that at least 30 people were killed and 58 others wounded.
"More details will be released to media later in the day after an ongoing investigation," he said.
The buses were reportedly carrying about 500 cadets and newly-graduates from a police training centre in neighbouring Wardak province to central Kabul.
The victims were travelling to join their families to celebrate Eid which marks the end of Ramadan or fasting month.
Several bystanders were among the killed and injured, witnesses said.
One bus was destroyed and four buses and several civilian vehicles were damaged in the deadly attack.
Afghan President Mohammad Ashraf Ghani strongly condemned the incident.
Ghani has ordered a thorough investigation to find how the decision on transportation of a large number of police graduates was made, according to a statement issued by the Presidential Palace.
The responsible officials must be held accountable if any dereliction is found in the incident, the statement said.
Indian Prime Minister Narendra Modi has condemned the "mindless attack".
"Anguished by the attack near Kabul. I condemn this mindless violence. My thoughts & prayers with the bereaved families & the injured," the Prime Minister tweeted.
Pakistan "strongly" condemned that attack and extended its "heartfelt condolences" to the Afghan government.
"Pakistan extends heartfelt condolences to the brotherly government and people of Afghanistan over the loss of innocent lives. Our hearts go out to the bereaved families and we wish speedy recovery for the injured," a Pakistan Foreign Office statement said.
The NATO-led Resolute Support mission in Afghanistan strongly condemned back to back explosions in the capital.
"RS strongly condemns the suicide attack in Kabul today. The Taliban continue to display a total disregard for human life," a statement by the Resolute Support said.
The statement added: "RS expresses its deepest condolences to the families of those killed and wounded in today’s attack.”
The Taliban-led insurgency has been rampant since early April when the militant group launched its annual rebel offensive in different areas of the country.
The Taliban urged civilians to stay away from official gatherings, military convoys and centres regarded as legitimate targets by the militants besides warning people not to support the government.
On June 10, a Taliban suicide attacker killed 14 people, including 12 Nepali security guards, and injured nine others in eastern Kabul.
