The Irish pop star reveals he got a fright during the band's concert in Boston, Massachusetts after two girls sneaked into his dressing room by posing as staff handing out towels.

Niall reveals the girls struck up a conversation with a group of his friends, who were unaware the pair was not supposed to be there.

"It was the last show in Boston... and my friends were over because we were going away on holiday," he tells Scottish radio station Clyde 1 Radio.

"I walked out to do an interview, came back into my dressing room and they're sitting there talking to these two girls who said they were dropping off towels. They were backstage just floating around, wearing T-shirts that looked like they were working at the arena... But then I walked in and the two of them freaked out. Everyone was like 'Woah' and they said 'It's okay, it's okay, we'll leave', but (we) were like 'You're not going anywhere yet, you're in trouble'."

Niall's bandmate Louis Tomlinson reveals British fans also put security guards to the test in London by mocking up fake passes for a show at the O2 Arena.

He says, "I think we were playing the O2 around the time of the (band's 2013 concert) film, and didn't some fans have some mock up AAA passes that they'd designed."