'Knights from the Islamic State' take credit for Tunisia national museum attack
An image made available on the jihadist website Welayat Salahuddin in June shows an IS militant waving the group's flag after allegedly seizing an Iraqi army checkpoint in the northern Iraqi province of Salahuddin. (AFP Photo/)

The Islamic State jihadist group on Thursday claimed responsibility for an attack on Tunisia's national museum that killed at least 21 people.

In an audio message posted online, IS said "two knights from the Islamic State... heavily armed with automatic weapons and grenades, targeted the Bardo Museum."

It threatened more attacks, saying: "What you have seen is only the start."

Twenty foreigners were among those killed when gunmen in military uniforms opened fire at visitors Wednesday as they got off a bus and then chased them inside the museum.

They included tourists from Japan, Spain, Colombia, Australia, Britain, Belgium, France, Poland and Italy, according to the Tunisian authorities.