TUNIS (AFP) - A Tunisian tour guide has told of how he used his knowledge of the Tunis museum attacked by gunmen Wednesday to help foreign tourists escape the carnage that left 20 others dead.

"I was on the second floor at around 11.30am , half an hour before it all started. Then I heard gunshots," Hamadi Ben Abdessalam, who has been a guide in the capital since 1970, told AFP Thursday.

"My first reaction was that this wasn't an attack. I told my clients that something had fallen from the ceiling but it was (they) who told me it was a terrorist attack," he said.

It was only when Abdessalam saw bullet casings that he realised how grave the situation was.

"We were all on our knees and everyone was panicking," he said.

The museum visitors were fortunate that Abdessalam, in his sixties and sporting a salt-and-pepper beard, knew the museum well.

"I started off to the right and 30 people followed me," he said. "Since I know the area... I headed in the direction of an emergency exit."

Wednesday's assault on the National Bardo Museum in central Tunis that killed 21 people - all except one of them foreign tourists - was claimed by the Islamic State in Iraq and Syria militant group, which promised further attacks.

"This is a first in Tunisia. No one saw this coming," Abdessalam said.

Nine of the slain tourists were from the MSC Splendida cruise ship, as was the group Abdessalam had been guiding.

He said he there were tears in his eyes when he arrived safely back with the Italians at the port of Goulette in northern Tunis after the attack.

"When we got to the port we experienced something extraordinary. We were applauded by the passengers, people embraced us, everyone was crying: the guides, the police," Abdessalam said.

"It was something really fantastic and it helped free us from the panic."