Afghanistan President Ashraf Ghani has condemned the "cowardly" truck bomb attack that ripped through the heart of Kabul's diplomatic district, killing at least 90 people and wounding hundreds.

The powerful explosion on Wednesday was described by officials as "one of the biggest" to have hit the Afghan capital.

The explosives were hidden in a tanker truck used to clean out septic systems, Najib Danish, deputy spokesman for the interior minister, said.

There was no immediate claim of responsibility. Zabihullah Mujahid, a spokesman for the Taliban, told Al Jazeera that the group was not involved in the attack.

Late on Wednesday, Afghanistan's spy agency blamed the Taliban-allied Haqqani network and neighbouring Pakistan for the blast.

"The plan for today's [Wednesday's] attack was drawn up by the Haqqani network with direct coordination and cooperation from Pakistan's Inter Services Intelligence (ISI)," the National Directorate of Security said in a statement.

Afghanistan has long accused Pakistan of supporting and sheltering Haqqani and Taliban commanders and fighters.

'Killing innocent people'

The blast gouged a crater about five metres deep near Zanbaq Square in the Wazir Akbar Khan district, where government buildings and foreign embassies are protected by a battery of their own security personnel, as well as Afghan police and National Security Forces.

"In this powerful attack 90 people have been killed and 400 wounded, including many women and children," said the government's media centre, with health officials warning the toll could climb higher.

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The victims appear mainly to have been Afghan civilians.

"The terrorists, even in the holy month of Ramadan, the month of goodness, blessing and prayer, are not stopping the killing of our innocent people," said Afghanistan President Ashraf Ghani.

Al Jazeera's Qais Azimy, reporting from Kabul, said the death toll could be as high as 100, with more than 500 wounded, according to sources.

"This is one of the biggest attacks in Afghanistan that I have ever seen," he said. "It's a very sad and very bloody day for the people of Kabul."

Aziz Navin, an IT engineer with the Afghan TOLOnews outlet, was one of the first victims to be named.

Mohammed Nazi, a driver with the BBC, was also killed, while four journalists from the broadcaster were being treated in hospital.

"[Nazir] was driving journalist colleagues to the office," the BBC said in a statement posted to Twitter. "He was in his late thirties and leaves a young family."

An Afghan security guard at the German embassy also died in the attack.

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Video shot at the scene showed burning debris, blasted walls and buildings and destroyed cars, many with dead or wounded people inside.

Al Jazeera's Azimy said the incident raised many security questions in Kabul, noting that many were wondering how "a truck full of explosives could manage to get to that highly secure" part of the capital.

Houses and shops hundreds of metres away from the blast site were damaged, with windows shattered and doors blown off their hinges.

"The explosion was so loud that it shattered all my windows. I did not hear something this big before," Fatima Faizi, a Kabul resident, told Al Jazeera.

Michael Kugelman of the US-based Wilson Center, told the Associated Press the deadly attack, signified an "intelligence failure" in Kabul.

"There was a clear failure to anticipate a major security threat in a highly secured area," he said.

"The fact that these intelligence failures keep happening suggest that something isn't working at the top, and major and urgent changes are needed in security policy," he said.

ISIL has claimed responsibility for several recent bombings in the Afghan capital, including a powerful blast targeting an armoured NATO convoy that killed at least eight people and wounded 28 on May 3.