A powerful bomb exploded in the morning rush hour in the centre of Kabul on Wednesday, killing at least 80 people, wounding hundreds and sending clouds of black smoke into the sky above the presidential palace and foreign embassies.

The bomb, one of the deadliest in Kabul and coming at the start of the holy month of Ramadan, exploded close to the fortified entrance to the German embassy, killing a security guard and wounding some staff, German Foreign Minister Sigmar Gabriel said on Twitter.

Basir Mujahid, a spokesman for Kabul police, said it was a car bomb near the German embassy. "But there are several other important compounds and offices near there too," he told Reuters.

The blast, which shattered windows and blew doors off their hinges in houses hundreds of metres (yards) away, was unusually strong, with some reports saying it was caused by explosives concealed in a water tanker.

A statement from the NATO-led Resolute Support (RS) mission in Kabul said Afghan security forces had prevented the vehicle from entering the heavily protected Green Zone that houses many foreign embassies as well as RS headquarters, suggesting it may not have reached its intended target.

A public health official said at least 80 people had been killed and more than 350 wounded. The victims appear mainly to have been Afghan civilians.

As well as the German embassy, the French and Chinese embassies were among those damaged, the two countries said, adding there were no immediate signs of injuries among their diplomats.

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

At the Wazir Akbar Khan hospital a few blocks away, there were scenes of chaos as ambulances brought in wounded and frantic relatives scanned casualty lists and questioned hospital staff for news.

"It felt like an earthquake," said 21-year-old Mohammad Hassan, describing the moment the blast struck the bank where he was working. His head wound had been bandaged but blood still soaked his white dress shirt.

TALIBAN DENY RESPONSIBILITY

Another lightly wounded victim, Nabib Ahmad, 27, said there was widespread destruction and confusion.

"I couldn't think clearly, there was a mess everywhere," he said.

Later, frenzy broke out outside the hospital as ambulances and police trucks began bringing in the bodies of those killed. Some bodies were burned or destroyed beyond recognition.

Indian Prime Minister Narendra Modi condemned the blast.

"India stands with Afghanistan in fighting all types of terrorism. Forces supporting terrorism need to be defeated," he said in a tweet.

The Taliban denied responsibility and said they condemned attacks that have no legitimate target and killed civilians. Islamic State, the other main militant group active in Afghanistan, has carried out high profile attacks in Kabul, including an attack on a military hospital in March that killed more than 50 people.

However the attack provided another clear demonstration that Ramadan, which began at the weekend, would provide little respite from the violence across Afghanistan.

The Taliban have been stepping up their push to defeat the U.S.-backed government and reimpose Islamic law after their 2001 ouster in a Washington-backed invasion.

Since most international troops withdrew at the end of 2014, the Taliban have gained ground and now control or contest about 40 percent of the country, according to U.S. estimates, though President Ashraf Ghani's government holds all provincial centres.

U.S. President Donald Trump is due to decide soon on a recommendation to send 3,000 to 5,000 more troops to bolster the small NATO training force and U.S. counter-terrorism mission now totalling just over 10,000.

The commander of U.S. forces in Afghanistan, General John Nicholson, told a congressional hearing earlier this year that he needed several thousand more troops to help Afghan forces break a "stalemate" with the Taliban.