Eight people were killed and 11 injured when the driver of the truck hit people on a cycle path in Lower Manhattan.
A 29-year-old man, reported to be an Uzbek immigrant, was shot and injured by police as he left the vehicle.
US President Donald Trump has ordered the Department of Homeland Security to "step up" vetting measures in the wake of the attack.
A note was found inside referring to the Islamic State militant group, law enforcement sources were quoted as saying by US media.
Officials said it was a terror attack - the deadliest in the city since the 11 September attacks.
The deaths of the five Argentines were confirmed by the country's foreign ministry. They were part of a group of 10 friends in New York to celebrate the 30th anniversary of their graduation from a polytechnic college in the central city of Rosario, Argentine media said.
One of the men who died, steel firm owner Ariel Erlij, helped pay for the friends' trip, La Nación newspaper reported (in Spanish).
What witnesses saw and heard
Argentina's foreign ministry named the men - all aged 48 or 49 - as Hernan Diego Mendoza, Diego Enrique Angelini, Alejandro Damián Pagnucco, Ariel Erlij and Hernan Ferruchi.
Another of the friends, Martin Ludovico Marro, is being treated in hospital. Three days of mourning have been declared in Rosario.
A Belgian national was killed and three others injured, Belgium's foreign minister said.
Two other victims have not yet been identified.
How did the attack unfold?
New York Police Department (NYPD) Commissioner James O'Neill said:
- Shortly after 15:00 (19:00 GMT), a pick-up truck rented from retailer Home Depot struck cyclists and pedestrians while being driven along the West St-Houston St path for many blocks
- It then hit a school bus, injuring two adults and two children on board, and came to a halt
- The driver emerged holding what appeared to be two handguns and made a statement "consistent with a terror attack"
- He was shot in the abdomen by a police officer stationed near the scene - the suspect later underwent surgery for bullet wounds
- The police officer, named in the US media as Ryan Nash, 28, "stopped the carnage moments after it began"
- A paintball gun and a pellet gun were recovered from the scene
One witness, Eugene, told ABC Channel 7 that he saw the truck driving quickly down the cycle path alongside the West Side Highway at full speed as it hit a number of people.
He also reported hearing about nine or 10 shots.
Who is the suspect?
The NYPD said only that the attacker was a 29-year-old man.
US media named him as Sayfullo Saipov, who had reportedly lived in Tampa, Florida, and Paterson, New Jersey.
He arrived in America from Uzbekistan in 2010 and is believed to be a legal resident in the country. Car-share service Uber confirmed he had been working for them.
Who is the suspect?
Halloween parade goes on despite attack
Three officials said Mr Saipov had previously come to the federal authorities' attention because of an unrelated investigation, the New York Times reported.
Police records show he was arrested in Missouri last year over a traffic fine.
Uzbekistan's president, Shavkat Mirziyoyev, told Donald Trump his country was ready to "use all forces and resources" to help investigate the attack. Uzbek officials have not yet confirmed the identity or nationality of the attacker.
What has been the reaction?
President Trump took to Twitter, saying:
- "In NYC, looks like another attack by a very sick and deranged person. Law enforcement is following this closely. NOT IN THE U.S.A.!"
- "We must not allow ISIS [Islamic State] to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!"
- "My thoughts, condolences and prayers to the victims and families of the New York City terrorist attack. God and your country are with you!"
He also announced he had ordered the Extreme Vetting Program to be stepped up, without elaborating further.
Mr Trump first said he wanted "extreme vetting" of immigrants during his presidential campaign last August. As president, he introduced a ban on arrivals to the US from a number of mainly-Muslim countries - a move due before the Supreme Court in the coming weeks.
The American Civil Liberties Union civil rights group said the term extreme vetting was a "euphemism for discriminating against Muslims".
New York City Mayor Bill de Blasio said the incident was a "cowardly act of terror aimed at innocent civilians".
"We know that this action was intended to break our spirit. But we also know that New Yorkers are strong, New Yorkers are resilient and our spirit will never be moved by an act of violence and an act meant to intimidate us," he said.
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