Donald Trump has called for "decisive action" against the Taliban after a bomb hidden in an ambulance in Kabul killed at least 95 people.
The bomber drove the ambulance packed with explosives in a crowded area near the Afghan capital's old interior ministry building in the early afternoon.
Another 158 people were wounded in the attack, some critically, in one of the biggest blasts to hit Kabul in years.
The Taliban said they had carried out the bombing, ending a violent week in Afghanistan after 22 people were killed during a siege at the Intercontinental Hotel.
The US President called the attack "despicable".
"This murderous attack renews our resolve and that of our Afghan partners," Mr Trump said in a statement.
"Now, all countries should take decisive action against the Taliban and the terrorist infrastructure that supports them."
Earlier, US Secretary of State Rex Tillerson said the Taliban's use of an ambulance as a weapon "represents inhumane disregard for the people of Afghanistan and all those working to bring peace to the country, and is a violation of the most basic international norms."
"All countries who support peace in Afghanistan have an obligation to take decisive action to stop the Taliban's campaign of violence," he went on to say.
"There can be no tolerance for those who support or offer sanctuary to terrorist groups."
The bomber used the ambulance to pass through one checkpoint, saying he was taking a patient to a nearby hospital, the interior ministry spokesman Nasrat Rahimi said.
When he reached the second checkpoint, he blew up the vehicle.
Medical professionals were struggling to handle the number of casualties caused by the attack, with many hospitals unable to handle the numbers and laying the wounded in hospital gardens with their intravenous drips.
"It's a massacre," said Dejan Panic, the coordinator in Afghanistan for the Italian aid group Emergency, which operates a trauma hospital that had treated dozens of wounded.
The ambulance car-bomb is the worst terrorist attack to hit Kabul since a truck filled with explosives was detonated near the German embassy last May, killing 150 people.
Following that attack, security was tightened around potential targets in the city, much of which is now heavily fortified with high concrete blast walls and police checkpoints.
The level of security has prompted anger at how the bomber managed to pass through the checkpoints.
"Officials must be held responsible," said former deputy interior minister Mohammad Ayub Salangi.
More from Afghanistan
Afghan President Ashraf Ghani has expressed confidence that an aggressive military strategy, backed by the country's US allies, has pushed Taliban insurgents away from the country's cities.
Despite the US assistance in providing airstrikes against the Taliban, the militant group has continued to display its ability to execute deadly attacks, even in the centre of Kabul.