A third teenager has been arrested in connection with a plot to carry out a terror attack at a Taylor Swift concert in Austria.

Austrian home secretary Gerhard Karner said an 18-year-old Iraqi national had been taken into custody on Thursday evening in Vienna after allegedly being in contact with the 19-year-old main suspect.

Mr Karner said the third teenager is "not directly connected to the attack plans" but comes from the same social circle as the main suspect.

All three of Swift's planned shows at the Ernst Happel Stadium were cancelled on Wednesday by event organisers Barracuda Music after it said it had a duty to protect everyone's safety.

Officials in Vienna said on Thursday that the main suspect, who is said to have North Macedonian roots, wanted to use weapons outside the stadium to kill as many people as possible.

| A photo released by authorities of the main suspect. ic: AP

Omar Haijawi-Pirchner, head of the Directorate of State Security and Intelligence, said the suspect was "clearly radicalised in the direction of the Islamic State (IS) and thinks it is right to kill infidels".

The main suspect and the 18-year-old arrested on Friday both pledged "oaths of allegiance" to IS, officials said.

Addressing the plot on Friday, United Nations spokesperson, Alessandra Vellucci, said it was a "good demonstration that radicalisation of people through the use of social media is really a concern".

A 17-year-old has also been arrested. According to officials, the Austrian citizen, who has a Turkish and Croatian background, had started a job at the concert venue days before the Swift shows were cancelled over the terror threat.

No additional suspects are being sought. A 15-year-old was questioned by police, but not arrested.

Concert organisers in Austria said they had expected up to 65,000 fans inside the stadium at each concert and as many as 30,000 onlookers outside, where authorities said the suspect planned to strike.

Heartbroken fans gathered in the Austrian capital on Friday as they came to terms with missing the record-breaking Eras tour.

Fans - some of whom were pictured crying and hugging - clapped, danced and sung as they belted out Swift's songs and took selfies with each other.