BARCELONA, Spain -- A van veered onto a sidewalk and barreled down a busy pedestrian zone Thursday in Barcelona's picturesque Las Ramblas district, swerving from side to side as it mowed down tourists and residents and turned the popular European vacation promenade into a bloody killing zone. Thirteen people were killed and 100 were injured in what authorities called a terror attack.

Late Thursday, Spanish police have said they killed several people south of Barcelona in Cambrils, according to AP. Authorities say this was a second alleged terror attempt.

Fifteen of the victims were seriously injured, another 23 were moderately wounded and 42 others were being treated for slight injuries. They are all being cared for at various hospitals.

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A U.S. citizen suffered minor injuries, the U.S. State Department confirmed to CBS News.

One Australian woman, from New South Wales, is in the hospital in a "serious but stable condition," according Australian foreign minister Julie Bishop.

Victims were left sprawled in the street, spattered with blood or crippled by broken limbs. Others fled in panic, screaming or carrying young children in their arms.

Catalonia's regional president, Carles Puigdemont, told Barcelona broadcaster TV3: "Our priority is to save lives. And our second priority is the police investigation, to find the people responsible of this attack and anyone who has helped them directly or indirectly."

Authorities said a Belgian was among the dead and a Greek woman was among the injured. Germany's Foreign Ministry said it was checking reports that Germans were among the victims.

However, at least three German citizens were among those who died, according to a report by broadcaster ZDF which cited security sources.

According to Puigdemont, two suspects have been arrested in connection with the attack. Police say one suspect is a Spanish national from Melilla and the other is Moroccan. Neither of the suspects drove the van. Police said the driver is still on the run.

According to police, the van attack is "connected" to an explosion that occurred Wednesday night in a town south of the city in which one person died and injured several more.

Amaq News issued a statement on its Telegram channel that reads in part: "A security source to Amaq agency: The perpetrators of the Barcelona attack are soldiers of the Islamic State of Syria and Iraq (ISIS). They carried out the attack in response to the calls to target coalition countries."

On Twitter, Spanish Prime Minister Mariano Rajoy said he is in contact with authorities and the priority is to attend to the injured.

Rajoy announced that Thursday's incident was "jihadist terrorism," the Reuters news agency says.

"Today the fight against terrorism is the principal priority for free and open societies like ours. It is a global threat and the response has to be global," Rajoy told a news conference in Barcelona.

In a photograph shown by RTVE, at least five people were lying on the ground in the street Thursday afternoon and were apparently being helped by police and others. Videos of the scene recorded people screaming as they fled.

Reuters reported that two armed men entered a restaurant following the crash. Police said they located a second van connected to the attack in the town of Vic.

Spain's El Pais newspaper, citing police sources, says perpetrators of the van incident were holed up in a bar. Armed police ran down the streets and through a market, checking in stores and cafes, presumably in search of them.

Barcelona's TV3 reports police surrounded a bar called Rey de Istanbul. A man thought to be the driver of a van was surrounded by police. The Spanish passport of a person of Moroccan origin was found at the scene of the attack.

The police force for Spain's Catalonia region says troopers have shot and killed a man who was in a car that hit two officers at a traffic blockade in Barcelona. The Mossos d'Esquadra force did not indicate if the incident was related to the van attack in the city's Las Ramblas district on Thursday.

Police say a bomb squad was checking an area in the outskirts of Barcelona, near the site where the driver of car struck police officers.

Local media reports say a white Ford Focus ran over the officers and then was intercepted by police 1.9 miles away. That's where troopers shot one man dead.

Three days of mourning have been declared in response to the attack. A minute of silence will be held Friday in main square "to show that we are not scared," Barcelona's mayor, Ada Colau, said.

Catalan Emergency Services asked authorities to close train stations in the area close to the crash site.

Las Ramblas, a street of stalls and shops that cuts through the center of Barcelona, is one of the city's top tourist destinations. People walk down a wide, pedestrianized path in the center of the street, but cars can travel on either side.

Authorities are asking people not to go near the area.

Keith Fleming, an American who lives in Barcelona, was watching TV in his building just off Las Ramblas when he heard a noise and went out to his balcony.

"I saw women and children just running and they looked terrified," he said.

He said there was a bang -- possibly from someone rolling down a store shutter -- and more people ran by. Then police arrived and pushed everyone a full block away. Even people leaning out of doors were being told to go back inside, he said.

Fleming said regular police had their guns drawn and riot police were at the end of his block, which was now deserted.

"It's just kind of a tense situation," Fleming said. "Clearly people were scared."

Carol Augustin, a manager at La Palau Moja -- an 18th-century place on Las Ramblas that houses government offices and a tourism information center -- said the van passed right in front of the building.

"We saw everything. People started screaming and running into the office. It was such a chaotic situation. There were families with children. The police made us close the doors and wait inside," she said.

In the U.S., President Trump tweeted about the attack from his New Jersey golf club, where he is on a working vacation.

"The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you!" the president wrote.

The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you! — Donald J. Trump (@realDonaldTrump) August 17, 2017

Vice President Mike Pence echoed Mr. Trump's message on Twitter by saying,"Sickened by horrific terror attack in Barcelona. U.S. will work w/allies to find those responsible & bring justice. Praying for victims."

Sickened by horrific terror attack in Barcelona. U.S. will work w/ allies to find those responsible & bring to justice. Praying for victims. https://t.co/Z4YcbtwRcy — Vice President Pence (@VP) August 17, 2017

He later delivered remarks from Panama, saying, "The latest scenes of carnage and mayhem sicken us all."

"The latest scenes of carnage and mayhem sicken us all," VP Pence says of deadly #Barcelona attack: https://t.co/Okz2bxbVVN pic.twitter.com/5rBzH4fHLP — CBS News (@CBSNews) August 17, 2017

"Whatever inspired today's terror attack, the United States stands ready to assist the people of Spain and find and punish those responsible. On this dark day, our prayers and prayers of all the American people are with the victims, their families and the good people of Spain," Pence said.

During a joint news conference with Japanese officials, U.S. Secretary of State Rex Tillerson said the U.S. stood ready to assist Spanish authorities in their probe of the attack. He stressed Washington's commitment to hunting down "terrorists around the world."

"I'd like to start by acknowledging the incident in Barcelona, which has the hallmarks, it appears, of yet another terrorist attack," Tillerson said. "Terrorists around the world should know the United States and our allies are resolved to find you and bring you to justice."

The U.S. Department of Homeland Security issued a written statement in response to the attack that reads in part: "The Department is standing by to support our allies as they respond to and recover from this horrendous attack. Our thoughts and prayers are with the victims and those affected. We will not let terrorism become the new normal. Instead, such acts of violence only harden our resolve to fight back against violent extremists, bring them to justice and dismantle their networks."

Spain's royal palace has condemned the attack, calling the perpetrators "assassins, simply criminals who will not terrorize us." The royal palace's statement, which was posted on Twitter, said that "All of Spain is Barcelona. Las Ramblas will once again be for all."

Other world leaders -- including British Prime Minister Theresa May, Belgian Prime Minister Charles Michel and French President Emmanuel Macron -- have condemned the attack.

Vatican spokesperson Greg Burke says Pope Francis is praying for the victims and expressing his closeness to their families. The Argentine pope was "greatly worried" about the attack and is following developments closely. The Vatican has greatly increased security for Francis' public events to prevent similar truck assaults on crowds that throng to his weekly audiences and Sunday blessings in St. Peter's Square.

Cars, trucks and vans have been the weapon of choice in multiple extremist attacks in Europe in the last year.

The most deadly was the driver of a tractor-trailer who targeted Bastille Day revelers in the southern French city of Nice in July 2016, killing 86 people. In December 2016, 12 people died after a driver used a hijacked trick to drive into a Christmas market in Berlin.

There have been multiple attacks this year in London, where a man in a rented SUV plowed into pedestrians on Westminster Bridge, killing four people before he ran onto the grounds of Parliament and stabbed an unarmed police officer to death in March.

Four other men drove onto the sidewalk of London Bridge, unleashing a rampage with knives that killed eight people in June. Another man also drove into pedestrians leaving a London mosque later in June.