Five friends from Argentina celebrating the 30th anniversary of their graduation were among those killed when a rental truck was driven into pedestrians and cyclists on a bike path in New York.
The men were reportedly part of a group of 10 friends exploring Manhattan on bicycles when the truck struck in Lower Manhattan on Tuesday.
In response to the atrocity - the deadliest terror attack to strike New York since September 11 - president Donald Trump ordered the security services to "step up" vetting procedures.
Eight people were killed and at least 11 others injured after a man drove the rented Home Depot truck on to the path on West Street at around 3pm local time (7pm GMT).
The driver, who has been identified by law enforcement officials as Sayfullo Saipov, 29, from Uzbekistan, was shot by police after jumping out of the vehicle with what turned out to be two fake guns. He was taken to hospital and is in police custody. Officers said his injuries are not life-threatening.
Hours after the attack, the names of the first victims emerged as Argentina's foreign ministry said Hernan Diego Mendoza, Diego Enrique Angelini, Alejandro Damian Pagnucco, Ariel Erlij and Hernan Ferruchi had died.
The group were marking three decades since their graduation from Polytechnic College in the city of Rosario in central Argentina.
A sixth Argentinian man with the group, Martin Ludovico Marro, was taken to the Presbyterian hospital in Manhattan.
A Belgian national was also among the dead, the country's deputy prime minister said.
On Tuesday night, police investigating the rampage were seen surrounding a white Toyota minivan with Florida number plates in the car park of Home Depot in Passaic, New Jersey.
Saipov had allegedly rented the pick-up truck an hour before the rampage from the state before unleashing terror on the streets of New York.
He reportedly shouted "Allahu akbar".
New York mayor Bill de Blasio described the attack as "a particularly cowardly act of terror" that was "aimed at innocent civilians going about their daily lives".
The 29-year-old suspect had been working as an Uber driver after passing a background check, according to a spokeswoman for the app.
In a statement, the firm added: "We are horrified by this senseless act of violence. Our hearts are with the victims and their families. We have reached out to law enforcement to provide our full assistance."
President Donald Trump praised the actions of the first responders and said on Twitter that he had ordered the Department of Homeland Security to step up its already "extreme vetting" programme, adding: "Being politically correct is fine, but not for this!"
A number of other officials in both the US and Europe offered condolences on social media.
Argentina's president Mauricio Macri tweeted: "Profoundly moved by the tragic deaths this afternoon in NY. We are at the disposal of the families of the Argentinian victims."
Foreign minister Jorge Faurie said that his heart was with the family and friends of those who had died "at this moment of deep and inexplicable pain".
New York mayor Bill de Blasio described the attack as "a particularly cowardly act of terror" while governor Andrew Cuomo said it was a "lone wolf" attack, and there was no evidence it was part of a wider plot.
Meanwhile the UK Foreign Office said it was standing by to provide assistance but was not aware of any British citizens involved as yet.
Prime Minister Theresa May tweeted that she was "appalled by this cowardly attack", saying the UK stands with New York.
The US president also tweeted: "We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"
The terrorist group has not claimed responsibility for the attack.
Irish premier Leo Varadkar said the attack was vicious and heartless.
"Those who have been killed and injured were innocent victims going about their day, completely unaware of what was about to happen," he said.
"I would like to express our solidarity with New Yorkers, who have faced considerable adversity in the past. Our thoughts and prayers are with the families and friends of the victims."