Two people were arrested near Vienna on Tuesday as police hunted the accomplices of a gunman or gunmen who killed four people in what the government called an Islamist terrorist attack in the Austrian capital, the Austria Press Agency (APA) reported

Austria said on Tuesday that the only known gunman in the shooting rampage across Vienna that left four people dead was an Islamic State group supporter.

Two men and two women were killed in the attack, and 14 more people have been injured, six of them seriously so, an interior ministry spokesman said.

According to the Chinese Embassy in Austria, the deaths include one Austrian Chinese. Another injured Chinese is at a local hospital but not in critical condition.

The shocked nation began three days of mourning after what Austrian Chancellor Sebastian Kurz described as a "repulsive terror attack."

Flags were lowered to half-mast on public buildings across the country, and people observed a minute of silence at noon as church bells rang.

The first shots were heard at around 8 p.m. in the heart of the city, near a synagogue and the world-famous opera house, where people were out in bars and restaurants enjoying a final night out hours before Austria was to reimpose a novel coronavirus lockdown.

Austrian Interior Minister Karl Nehammer urged Vienna residents to remain in their homes and said children would not be expected to go to school on Tuesday.

Jewish community leader Oskar Deutsch said on Twitter that it was not clear whether the Vienna synagogue and adjoining offices had been the target and that they were closed at the time.

Frequent sirens and helicopters could be heard in the city center as emergency services responded to the attack.

Kurz condemned the "hideous terrorist attack" in the Austrian capital, vowing to "take decisive action" against the perpetrators.

"We are currently going through difficult times in our republic," Kurz wrote on Twitter. "We will never allow ourselves to be intimidated by terrorism and will fight these attacks resolutely by all means."

What we know about the suspect(s) so far

One of the suspected killers, who was shot dead by police after Monday's attack, was a 20-year-old Islamic State group sympathizer with dual Austrian and Macedonian nationality, Nehammer said.

He had been convicted of a terror offense in April last year for trying to travel to Syria, Nehammer told the APA news agency.

Nehammer told media at a press conference earlier on Tuesday that police had used explosives to blast their way into the apartment of the dead suspect who had been "heavily armed."

"All the signs make it clear it's a radicalized person and a person who feels closely connected to IS," Nehammer said.

There were six different shooting locations in the immediate vicinity of the street housing the central synagogue, and the attackers used long guns.

According to Nehammer, police are searching for more possible assailants who may still be at large, and several neighboring countries have stepped up border checks.

"It's difficult for us at the moment to define whether the attack was carried out by one perpetrator or more than one," said Gerhard Puerstl of the Vienna police. 

World leaders condemn Vienna shootings

Austria had until now been spared the sort of major attacks that have hit other European countries. Monday's shooting followed a spate of Islamist attacks in France and triggered an outpouring of solidarity from world leaders.

European Council President Charles Michel, European Commission President Ursula von der Leyen and leaders of European countries condemned the shooting attacks in Vienna late on Monday.

"Europe strongly condemns this cowardly act that violates life and our human values," said Michel on Twitter. "We stand with Austria."

Von der Leyen said she was shocked and saddened by the brutal attack that took place in Vienna. "Europe stands in full solidarity with Austria. We are stronger than hatred and terror."

French President Emmanuel Macron said on Monday evening that the French share the shock and sorrow of the Austrian people following the shootings, pledging that "our enemies must know who they are dealing with. We will not give in."

The German Foreign Office tweeted late Monday, "Even we don't know the full extent of the terror yet, our thoughts are with the injured and victims at this difficult time. We will not give way to hatred which is aimed at dividing our societies."

The country has also stepped up checks at its border with Austria following a manhunt underway for an assailant still at large, police told AFP Tuesday. Tightened controls at the border are now a "tactical priority" for the federal police force, a spokesman for German federal police said.

In London, British Prime Minister Johnson said the "UK's thoughts are with the people of Austria – we stand united with you against terror."

Italian Prime Minister Giuseppe Conte tweeted, "There is no room for hatred and violence in our common European home."

Dutch Prime Minister Mark Rutte called the shooting "a heinous act" and expressed "solidarity" with Austria.

Russian President Vladimir Putin called the Vienna shooting a "cruel and cynical" crime in a condolences telegram to the Austrian leadership.

(With input from agencies)