All the BBC's websites were unavailable for several hours on Thursday after what a BBC source described as a distributed denial of service (DdoS) attack.
A DdoS attack is an attempt to force an online service unavailable by overwhelming it with traffic from multiple sources.
The BBC has not confirmed or denied such an attack caused the problems. The corporation's press office said the BBC would not be commenting on the group's claim.
"It was only a test, we didn't exactly plan to take it down for multiple hours. Our servers are quite strong," New World Hacking said.
In a tweet to BBC technology correspondent Rory Cellan-Jones, the group said: "We are based in the US, but we strive to take down ISIS [IS] affiliated websites, also ISIS members.
"We realise sometimes what we do is not always the right choice, but without cyber hackers... Who is there to fight off online terrorists? "The reason we really targeted [the] BBC is because we wanted to see our actual server power."
The BBC's main website is the 89th biggest in the world, according to web analytics firm Alexa, and is the seventh-ranked site in the UK.
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