KABUL — A suicide attacker struck the fortified heart of the Afghan capital with a massive truck bomb Wednesday, killing 90 people, wounding 400, and raising new fears about the government’s ability to protect its citizens nearly 16 years into a war with insurgents.

The bomber drove into Kabul’s heavily guarded diplomatic quarter during the morning rush hour, leaving behind a bloody scene of chaos and destruction in one of the worst attacks since the drawdown of foreign forces from Afghanistan in 2014.

Most of the casualties were civilians, including children, said Ismail Kawasi, spokesman of the public health ministry. But the dead also included Afghan security guards at the facilities, including the US Embassy, while 11 American contractors were wounded — none with life-threatening injuries, a US State Department official said.

‘‘I have been to many attacks, taken wounded people out of many blast sites, but I can say I have ever seen such a horrible attack as I saw this morning,’’ ambulance driver Alef Ahmadzai said. ‘‘Everywhere was on fire and so many people were in critical condition.’’

There was no claim of responsibility for the attack, which came in the first week of the Muslim holy month of Ramadan. The Taliban flatly denied any involvement in an e-mail to news outlets and condemned all attacks against civilians.

The explosives were hidden in a tanker truck used to clean out septic systems, said Najib Danish, deputy spokesman for the interior minister. The number of dead and wounded was provided by the Afghan government’s media center, citing a statement from the Afghan Ulema Council, the country’s top religious body.

The blast gouged a crater about 15 feet deep near Zanbaq Square in the Wazir Akbar Khan district, where foreign embassies are protected by a battery of their own security personnel as well as Afghan police and National Security Forces. The nearby German Embassy was heavily damaged.

Also in the area is Afghanistan’s Foreign Ministry, the Presidential Palace, and its intelligence and security headquarters, guarded by soldiers trained by the US and its coalition partners.

‘‘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 President Ashraf Ghani.

President Trump spoke with Ghani after the attack, and Secretary of State Rex Tillerson condemned it as a ‘‘senseless and cowardly act.’’

‘‘The United States stands with the government and the people of Afghanistan and will continue to support their efforts to achieve peace, security, and prosperity for their country,’’ Tillerson said in a statement.

Afghanistan’s war, the longest ever involving US troops, has shown no sign of letting up, and the introduction into the battle of an Islamic State affiliate has made the country only more volatile.

Although they are small in number, militants from the Islamic State in Khorasan — an ancient name for parts of Afghanistan, Iran, and Central Asia — have taken credit for several brazen assaults on the capital.

‘‘Let’s be clear: This is an intelligence failure, as has been the case with so many other attacks in Kabul and beyond. There was a clear failure to anticipate a major security threat in a highly secured area,’’ said Michael Kugelman of the US-based Wilson Center.

‘‘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 by e-mail.

Still, there are questions about whether a US pledge to send more troops to Afghanistan will curb the violence.

‘‘The sad reality is that more foreign troops would not necessarily ensure these attacks happen less,’’ Kugelman said. ‘‘But they could help by supplementing training programs meant to enhance Afghan intel collection capacities, which have long been a deficiency in Afghanistan.’’

There are currently 8,500 US troops in Afghanistan with a promise of more to come.

Afghan lawmaker and analyst Nasrullah Sadeqizada bemoaned the abysmal security, saying ‘‘the situation is deteriorating day by day.’’

In an interview, Sadeqizada criticized US and NATO troops in Afghanistan, saying they have done little to improve protection.

‘‘If the situation continues to deteriorate, Afghans will lose all trust in the foreigners who are in Afghanistan as friends,’’ he warned.

General Mirza Mohammad Yarmand, former deputy interior minister, said more troops won’t help, although he urged the global community to stay committed to Afghanistan.

‘‘I don’t think that more US or NATO soldiers can solve the security problems in Afghanistan,’’ he said.

Material from the Washington Post was used in this report.